* gnu/services/base.scm (file-system-service): New procedure.
(user-processes-service): Add 'requirements' parameter.
* gnu/services/dmd.scm (dmd-configuration-file): Use (guix build
linux-initrd).
* guix/build/linux-initrd.scm (guix): Export 'check-file-system'.
* gnu/system.scm (file-union): New procedure.
(essential-services): Use it. Add that to the returned list.
* gnu/services/dmd.scm (dmd-configuration-file): Add derivations for
the (guix build syscalls) module, and add that to the load path of
dmd.conf.
* gnu/services/base.scm (root-file-system-service): Rewrite using the
'sync' and 'mount' procedures.
* gnu/services.scm (<service>)[inputs]: Remove.
* gnu/system.scm (links): Remove.
(etc-directory): Add PASSWD and SHADOW to #:inputs.
(operating-system-boot-script): Pass ETC to 'dmd-configuration-file'.
(operating-system-derivation): Remove EXTRAS from the union.
* gnu/system/linux.scm (pam-service->configuration): Rewrite in terms of
'gexp->derivation'. Compute the contents on the build side. Expect
'arguments' to contain a list of gexps.
(pam-services->directory): Rewrite in terms of 'gexp->derivation'.
(unix-pam-service): Change 'arguments' to a list of one gexp.
* gnu/system/shadow.scm (<user-account>)[inputs]: Remove.
[shell]: Change default value to a gexp.
(passwd-file): Rewrite in terms of 'gexp->derivation'. Compute
contents on the build side.
* gnu/services/base.scm (host-name-service, mingetty-service,
nscd-service, syslog-service, guix-service): Change 'start' and 'stop'
to gexps; remove 'inputs' field.
(guix-build-accounts): Change 'shell' field to a gexp.
* gnu/services/networking.scm (static-networking-service): Change
'start' and 'stop' to gexps; remove 'inputs' field.
* gnu/services/xorg.scm (slim-service): Likewise.
* gnu/services/dmd.scm (dmd-configuration-file): Expect ETC to be a
derivation. Change 'config' to a gexp. Use 'gexp->file' instead of
'text-file'.
* doc/guix.texi (Defining Services): Update nscd example with gexps, and
without 'inputs'. Add xref to "G-Expressions".
* gnu/services/base.scm (%base-services): New variable.
* gnu/system.scm (<operating-system>)[services]: Change the default
value to %BASE-SERVICES.
* doc/guix.texi (Using the Configuration System): Change
'%standard-services' to '%base-services'.
