Add (gnu system file-systems).

This fixes a circular dependency between (gnu system) and (gnu system
linux-initrd), where the latter could end up being compiled before
'file-system-type' was defined as a macro.

* gnu/system.scm (<file-system>, %fuse-control-file-system,
  %binary-format-file-system): Move to...
* gnu/system/file-systems.scm: ... here.  New file.
* build-aux/hydra/demo-os.scm, gnu/system/linux-initrd.scm,
  gnu/system/vm.scm: Use it.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.

gnu/system/file-systems.scm

@@ -0,0 +1,72 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu system file-systems)
+  #:use-module (guix records)
+  #:export (<file-system>
+            file-system
+            file-system?
+            file-system-device
+            file-system-mount-point
+            file-system-type
+            file-system-needed-for-boot?
+            file-system-flags
+            file-system-options
+
+            %fuse-control-file-system
+            %binary-format-file-system))
+
+;;; Commentary:
+;;;
+;;; Declaring file systems to be mounted.
+;;;
+;;; Code:
+
+;; File system declaration.
+(define-record-type* <file-system> file-system
+  make-file-system
+  file-system?
+  (device           file-system-device)           ; string
+  (mount-point      file-system-mount-point)      ; string
+  (type             file-system-type)             ; string
+  (flags            file-system-flags             ; list of symbols
+                    (default '()))
+  (options          file-system-options           ; string or #f
+                    (default #f))
+  (needed-for-boot? file-system-needed-for-boot?  ; Boolean
+                    (default #f))
+  (check?           file-system-check?            ; Boolean
+                    (default #t)))
+
+(define %fuse-control-file-system
+  ;; Control file system for Linux' file systems in user-space (FUSE).
+  (file-system
+    (device "fusectl")
+    (mount-point "/sys/fs/fuse/connections")
+    (type "fusectl")
+    (check? #f)))
+
+(define %binary-format-file-system
+  ;; Support for arbitrary executable binary format.
+  (file-system
+    (device "binfmt_misc")
+    (mount-point "/proc/sys/fs/binfmt_misc")
+    (type "binfmt_misc")
+    (check? #f)))
+
+;;; file-systems.scm ends here