bug#60246: Inability to add pseudo-filesystem fstab entries

[mirai <mirai@makinata.eu>]

An entry of the form:

--8<---------------cut here---------------start------------->8---
  (file-system
    (mount-point "/media/foo-mount")
    (create-mount-point? #t)
    (mount? #f)
    (device "none")
    (type "overlay")
    (flags '(no-atime no-dev no-suid no-exec read-only))
    (options (string-append "lowerdir="
                            (string-join '("/srv/foo/overlays/bar"
                                           "/srv/foo/overlays/baz") ":")))
    (check? #f))
--8<---------------cut here---------------end--------------->8---

Does not result in a fstab entry line, which makes it impossible to mount. According to Guix docs, this shouldn't be the case:

--8<---------------cut here---------------start------------->8---
mount? (default: #t)

    This value indicates whether to automatically mount the file system when the system is brought up. When set to #f, the file system gets an entry in /etc/fstab (read by the mount command) but is not automatically mounted.
--8<---------------cut here---------------end--------------->8---

Looking at gnu/services/base.scm there's this snippet:

--8<---------------cut here---------------start------------->8---
(define (file-system-fstab-entries file-systems)
  "Return the subset of @var{file-systems} that should have an entry in
@file{/etc/fstab}."
  ;; /etc/fstab is about telling fsck(8), mount(8), and umount(8) about
  ;; relevant file systems they'll have to deal with.  That excludes "pseudo"
  ;; file systems.
  ;;
  ;; In particular, things like GIO (part of GLib) use it to determine the set
  ;; of mounts, which is then used by graphical file managers and desktop
  ;; environments to display "volume" icons.  Thus, we really need to exclude
  ;; those pseudo file systems from the list.
  (remove (lambda (file-system)
            (or (member (file-system-type file-system)
                        %pseudo-file-system-types)
                (memq 'bind-mount (file-system-flags file-system))))
          file-systems))
--8<---------------cut here---------------end--------------->8---

That seems to remove such pseudo-fs entries, regardless of 'mount?' value.
This is not a "documentation" bug, as there are valid uses for having pseudo-fs entries.

Right now I'm trying to add an overlayfs mount that is layered over a NFS system and
Guix doesn't exactly deal very well with mounting NFS as it depends on networking. To sidestep this,
I am mounting the NFS volume through a simple-service that puts a dependency on networking.

From here the fstab problem begins: since it can't be mounted at boot "conventionally", it is set
as mount? #f with the intention to mount it later via "mount /media/foo-mount" after NFS volume is ready
but since this entry does not end up in /etc/fstab, there's no way to mount it.