Further tidyup of nfs_umount.

- remove non-used arguments from del_mtab.
- Don't try to pass a "host:/path" string to umount.
  It used to be possible to umount("/dev/whatever"). It has never
  been possible to umount("host:/path").
- Don't try to read /proc/mounts first.  Some mount options (mount_vers)
  are only stored in /etc/mtab, not in /proc/mounts.  So we have to
  prefer /etc/mtab as getmntXbackwards do.
- Only every call one of getmnt{dir,dev}backwards, depending on whether
  'spec' looks like a path name or a host:/path.
- Don't call _nfsumount unless we have a host:/path, or del_mtab unless we
  have a path name.