doc: Replace incorrect uses of @var with @code.

@var is for meta-syntactic variables but it is used in some places for
mere variables, which is incorrect and leads to inconsistent layout in
PDF and HTML.

* doc/guix.texi (package Reference, The Store Monad)
(G-Expressions, operating-system Reference)
(File Systems, Base Services, Log Rotation)
(Networking Services, Invoking guix deploy): Replace incorrect uses of
@var with @code.

doc/guix.texi

@@ -5817,7 +5817,7 @@ or a list of such values.
 @item @code{home-page}
 The URL to the home-page of the package, as a string.
 
-@item @code{supported-systems} (default: @var{%supported-systems})
+@item @code{supported-systems} (default: @code{%supported-systems})
 The list of systems supported by the package, as strings of the form
 @code{architecture-kernel}, for example @code{"x86_64-linux"}.
 
@@ -7380,7 +7380,7 @@ increments the current state value:
 @result{} 3
 @end lisp
 
-When ``run'' through @var{%state-monad}, we obtain that additional state
+When ``run'' through @code{%state-monad}, we obtain that additional state
 value, which is the number of @code{square} calls.
 @end defvr
 
@@ -7412,7 +7412,7 @@ The main interface to the store monad, provided by the @code{(guix
 store)} module, is as follows.
 
 @defvr {Scheme Variable} %store-monad
-The store monad---an alias for @var{%state-monad}.
+The store monad---an alias for @code{%state-monad}.
 
 Values in the store monad encapsulate accesses to the store.  When its
 effect is needed, a value of the store monad must be ``evaluated'' by
@@ -7773,7 +7773,7 @@ information about monads.)
        [#:system (%current-system)] [#:target #f] [#:graft? #t] @
        [#:hash #f] [#:hash-algo #f] @
        [#:recursive? #f] [#:env-vars '()] [#:modules '()] @
-       [#:module-path @var{%load-path}] @
+       [#:module-path @code{%load-path}] @
        [#:effective-version "2.2"] @
        [#:references-graphs #f] [#:allowed-references #f] @
        [#:disallowed-references #f] @
@@ -8077,7 +8077,7 @@ item.  This is achieved using the @code{lower-object} monadic procedure.
 
 @deffn {Monadic Procedure} lower-object @var{obj} [@var{system}] @
            [#:target #f]
-Return as a value in @var{%store-monad} the derivation or store item
+Return as a value in @code{%store-monad} the derivation or store item
 corresponding to @var{obj} for @var{system}, cross-compiling for
 @var{target} if @var{target} is true.  @var{obj} must be an object that
 has an associated gexp compiler, such as a @code{<package>}.
@@ -11252,7 +11252,7 @@ By that, we mean all the global system configuration, not per-user
 configuration (@pxref{Using the Configuration System}).
 
 @table @asis
-@item @code{kernel} (default: @var{linux-libre})
+@item @code{kernel} (default: @code{linux-libre})
 The package object of the operating system kernel to use@footnote{Currently
 only the Linux-libre kernel is supported.  In the future, it will be
 possible to use the GNU@tie{}Hurd.}.
@@ -11409,11 +11409,11 @@ As a user you should @emph{never} need to touch this field.
 Linux @dfn{pluggable authentication module} (PAM) services.
 @c FIXME: Add xref to PAM services section.
 
-@item @code{setuid-programs} (default: @var{%setuid-programs})
+@item @code{setuid-programs} (default: @code{%setuid-programs})
 List of string-valued G-expressions denoting setuid programs.
 @xref{Setuid Programs}.
 
-@item @code{sudoers-file} (default: @var{%sudoers-specification})
+@item @code{sudoers-file} (default: @code{%sudoers-specification})
 @cindex sudoers file
 The contents of the @file{/etc/sudoers} file as a file-like object
 (@pxref{G-Expressions, @code{local-file} and @code{plain-file}}).
@@ -11579,7 +11579,7 @@ variables.
 
 @defvr {Scheme Variable} %base-file-systems
 These are essential file systems that are required on normal systems,
-such as @var{%pseudo-terminal-file-system} and @var{%immutable-store} (see
+such as @code{%pseudo-terminal-file-system} and @code{%immutable-store} (see
 below.)  Operating system declarations should always contain at least
 these.
 @end defvr
@@ -12646,7 +12646,7 @@ and caches.
 @defvr {Scheme Variable} %nscd-default-configuration
 This is the default @code{<nscd-configuration>} value (see below) used
 by @code{nscd-service}.  It uses the caches defined by
-@var{%nscd-default-caches}; see below.
+@code{%nscd-default-caches}; see below.
 @end defvr
 
 @deftp {Data Type} nscd-configuration
@@ -12671,7 +12671,7 @@ Name of the nscd log file.  This is where debugging output goes when
 Integer denoting the debugging levels.  Higher numbers mean that more
 debugging output is logged.
 
-@item @code{caches} (default: @var{%nscd-default-caches})
+@item @code{caches} (default: @code{%nscd-default-caches})
 List of @code{<nscd-cache>} objects denoting things to be cached; see
 below.
 
@@ -12943,7 +12943,7 @@ well as in the @var{groups} field of the @var{operating-system} record.
 @end lisp
 
 @defvr {Scheme Variable} urandom-seed-service-type
-Save some entropy in @var{%random-seed-file} to seed @file{/dev/urandom}
+Save some entropy in @code{%random-seed-file} to seed @file{/dev/urandom}
 when rebooting.  It also tries to seed @file{/dev/urandom} from
 @file{/dev/hwrng} while booting, if @file{/dev/hwrng} exists and is
 readable.
@@ -13322,7 +13322,7 @@ Either @code{#f} or a gexp to execute once the rotation has completed.
 @end deftp
 
 @defvr {Scheme Variable} %default-rotations
-Specifies weekly rotation of @var{%rotated-files} and of
+Specifies weekly rotation of @code{%rotated-files} and of
 @file{/var/log/guix-daemon.log}.
 @end defvr
 
@@ -13782,7 +13782,7 @@ clock synchronized with that of the given servers.
 
 @defvr {Scheme Variable} %openntpd-servers
 This variable is a list of the server addresses defined in
-@var{%ntp-servers}.
+@code{%ntp-servers}.
 @end defvr
 
 @deftp {Data Type} openntpd-configuration
@@ -13800,7 +13800,7 @@ See @uref{https://man.openbsd.org/ntpd.conf, upstream documentation} for more
 information.
 @item @code{server} (default: @code{'()})
 Specify a list of IP addresses or hostnames of NTP servers to synchronize to.
-@item @code{servers} (default: @var{%openntp-servers})
+@item @code{servers} (default: @code{%openntp-servers})
 Specify a list of IP addresses or hostnames of NTP pools to synchronize to.
 @item @code{constraint-from} (default: @code{'()})
 @code{ntpd} can be configured to query the ‘Date’ from trusted HTTPS servers via TLS.
@@ -27109,8 +27109,8 @@ evaluates to.  As an example, @var{file} might contain a definition like this:
 
 The file should evaluate to a list of @var{machine} objects.  This example,
 upon being deployed, will create a new generation on the remote system
-realizing the @code{operating-system} declaration @var{%system}.
-@var{environment} and @var{configuration} specify how the machine should be
+realizing the @code{operating-system} declaration @code{%system}.
+@code{environment} and @code{configuration} specify how the machine should be
 provisioned---that is, how the computing resources should be created and
 managed.  The above example does not create any resources, as a
 @code{'managed-host} is a machine that is already running the Guix system and