umount.nfs: fix nfs4 check

umount.nfs: fix nfs4 check

From: Ian Kent <ikent@redhat.com>

nfs_umount_is_vers4() doesn't take acount of the escaping of characters
seen in /proc/mounts and /etc/mtab as the functions in fstab.c do. This
leads to an inability to umount a mount containing any of these escaped
characters (like spaces).

This patch changes nfs_umount_is_vers4() to use functions in fstab.c and
adds a function to fstab.c to read /proc/mounts specifically, as it was
used  for the check in nfs_umount_is_vers4() previously.

Signed-off-by: Ian Kent <ikent@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>