gnu: Consistently Write ‘file system(s)’.

It is the GNU way.

* doc/guix.texi (Build Systems, DNS Services): Write ‘file system(s)’.
* gnu/build/vm.scm (create-ext-file-system, create-fat-file-system):
Likewise.
* gnu/packages/backup.scm (dirvish, rsnapshot)[description]: Likewise.
* gnu/packages/check.scm (python-testpath)[description]: Likewise.
* gnu/packages/disk.scm (pydf)[description]: Likewise.
* gnu/packages/file-systems.scm (disorderfs)[synopsis, description]: Likewise.
(glusterfs)[description]: Likewise.
* gnu/packages/haskell.scm (ghc-directory, ghc-system-fileio-bootstrap)
(ghc-system-fileio)[synopsis]: Likewise.
(ghc-fsnotify)[description]: Likewise.
* gnu/packages/linux.scm (proot)[description]: Likewise.
(jmtpfs)[synopsis, description]: Likewise.
* gnu/packages/mate.scm (caja, caja-extensions)[description]: Likewise.
* gnu/packages/storage.scm (ceph)[description]: Likewise.
* gnu/packages/sync.scm (lsyncd)[description]: Likewise.
* gnu/packages/syncthing.scm (syncthing)[synopsis]: Likewise.
(go-github-com-zillode-notify)[description]: Likewise.
* gnu/services/nfs.scm (pipefs-service-type): Likewise.
* guix/scripts/system.scm (perform-action): Likewise.