system: Provide declarations for the 'fusectl' and 'binfmt_misc' file systems.

* gnu/system.scm (%fuse-control-file-system,
  %binary-format-file-system): New variables.
* build-aux/hydra/demo-os.scm (file-systems): New field.

build-aux/hydra/demo-os.scm

@@ -43,6 +43,11 @@
  (host-name "gnu")
  (timezone "Europe/Paris")
  (locale "en_US.UTF-8")
+ (file-systems
+  ;; We don't provide a file system for /, but that's OK because the VM build
+  ;; code will automatically declare the / file system for us.
+  (list %fuse-control-file-system
+        %binary-format-file-system))
  (users (list (user-account
                (name "guest")
                (group "wheel")

gnu/system.scm

@@ -64,7 +64,10 @@ (define-module (gnu system)
             file-system-type
             file-system-needed-for-boot?
             file-system-flags
-            file-system-options))
+            file-system-options
+
+            %fuse-control-file-system
+            %binary-format-file-system))
 
 ;;; Commentary:
 ;;;
@@ -126,6 +129,11 @@ (define-record-type* <operating-system> operating-system
   (sudoers operating-system-sudoers               ; /etc/sudoers contents
            (default %sudoers-specification)))
 
+
+;;;
+;;; File systems.
+;;;
+
 ;; File system declaration.
 (define-record-type* <file-system> file-system
   make-file-system
@@ -142,6 +150,22 @@ (define-record-type* <file-system> file-system
   (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)))
+
 
 ;;;
 ;;; Derivation.