file-systems: Support the 'remount' mount flag.

This allows remounting file systems which are already mounted by
the initrd with custom mount options (e.g. /proc with hidepid=2).

* gnu/build/file-systems.scm (mount-flags->bit-mask): Map 'remount
to MS_REMOUNT.
* gnu/system/file-systems.scm (invalid-file-system-flags): Add
'remount to the list of KNOWN-FLAGS.
* doc/guix.texi (File Systems): Document it.

Fixes: guix/guix#3849
Change-Id: I0ab116a5b4f7bd201e955ecf022e260c63828dc4
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -18443,7 +18443,7 @@ bits), @code{no-atime} (do not update file access times),
 @code{no-diratime} (likewise for directories only),
 @code{strict-atime} (update file access time), @code{lazy-time} (only
 update time on the in-memory version of the file inode),
-@code{no-exec} (disallow program execution), and @code{shared} (make the
+@code{no-exec} (disallow program execution), @code{remount} (remount a mounted file system), and @code{shared} (make the
 mount shared).
 @xref{Mount-Unmount-Remount,,, libc, The GNU C Library Reference
 Manual}, for more information on these flags.

gnu/build/file-systems.scm

@@ -1312,6 +1312,8 @@ corresponds to the symbols listed in FLAGS."
        (logior MS_STRICTATIME (loop rest)))
       (('lazy-time rest ...)
        (logior MS_LAZYTIME (loop rest)))
+      (('remount rest ...)
+       (logior MS_REMOUNT (loop rest)))
       (('shared rest ...)
        (loop rest))
       (()

gnu/system/file-systems.scm

@@ -123,7 +123,7 @@
     ;; Note: Keep in sync with 'mount-flags->bit-mask'.
     (let ((known-flags '(read-only
                          bind-mount no-suid no-dev no-exec
-                         no-atime no-diratime strict-atime lazy-time
+                         no-atime no-diratime strict-atime lazy-time remount
                          shared)))
       (lambda (flags)
         "Return the subset of FLAGS that is invalid."