remote-fs.target: want remote-fs-pre.target

This pulls in remote-fs-pre.target if remote-fs.target is needed.

Previously remote-fs-pre.target was not active, if no remote fs was
mounted from /etc/fstab. So, every manual remote fs mount was ordered
against the inactive remote-fs-pre.target and umount.target.

Because remote-fs-pre.target was not active, the remote fs was umounted
at umount.target time, which was too late (network already down).

Now remote-fs-pre.target is active, even if no remote fs is mounted.
On shutdown it is deactivated in the correct order and all manual remote
fs mounts also.