system: Add 'create-mount-point?' file system option.

* gnu/system/file-systems.scm (<file-system>)[create-mount-point?]: New
  field.
* gnu/services/base.scm (file-system-service): Add #:create-mount-point?
  parameter and honor it.
* gnu/system.scm (other-file-system-services): Update
  'file-system-service' call accordingly.
* doc/guix.texi (File Systems): Document it.

doc/guix.texi

@@ -3054,6 +3054,9 @@ instance, for the root file system.
 This Boolean indicates whether the file system needs to be checked for
 errors before being mounted.
 
+@item @code{create-mount-point?} (default: @code{#f})
+When true, the mount point is created if it does not exist yet.
+
 @end table
 @end deftp
 

gnu/services/base.scm

@@ -96,11 +96,13 @@ (define (root-file-system-service)
       (respawn? #f)))))
 
 (define* (file-system-service device target type
-                              #:key (check? #t) options (title 'any))
+                              #:key (check? #t) create-mount-point?
+                              options (title 'any))
   "Return a service that mounts DEVICE on TARGET as a file system TYPE with
 OPTIONS.  TITLE is a symbol specifying what kind of name DEVICE is: 'label for
 a partition label, 'device for a device file name, or 'any.  When CHECK? is
-true, check the file system before mounting it."
+true, check the file system before mounting it.  When CREATE-MOUNT-POINT? is
+true, create TARGET if it does not exist yet."
   (with-monad %store-monad
     (return
      (service
@@ -109,6 +111,9 @@ (define* (file-system-service device target type
       (documentation "Check, mount, and unmount the given file system.")
       (start #~(lambda args
                  (let ((device (canonicalize-device-spec #$device '#$title)))
+                   #$(if create-mount-point?
+                         #~(mkdir-p #$target)
+                         #~#t)
                    #$(if check?
                          #~(begin
                              ;; Make sure fsck.ext2 & co. can be found.

gnu/system.scm

@@ -181,10 +181,11 @@ (define file-systems
   (sequence %store-monad
             (map (match-lambda
                   (($ <file-system> device title target type flags opts
-                                    #f check?)
+                                    #f check? create?)
                    (file-system-service device target type
                                         #:title title
                                         #:check? check?
+                                        #:create-mount-point? create?
                                         #:options opts)))
                  file-systems)))
 

gnu/system/file-systems.scm

@@ -28,6 +28,8 @@ (define-module (gnu system file-systems)
             file-system-needed-for-boot?
             file-system-flags
             file-system-options
+            file-system-check?
+            file-system-create-mount-point?
 
             %fuse-control-file-system
             %binary-format-file-system
@@ -57,7 +59,9 @@ (define-record-type* <file-system> file-system
   (needed-for-boot? file-system-needed-for-boot?  ; Boolean
                     (default #f))
   (check?           file-system-check?            ; Boolean
-                    (default #t)))
+                    (default #t))
+  (create-mount-point? file-system-create-mount-point? ; Boolean
+                       (default #f)))
 
 (define %fuse-control-file-system
   ;; Control file system for Linux' file systems in user-space (FUSE).