umount: call /sbin/umount.<uhelper> for root user too

umount: call /sbin/umount.<uhelper> for root user too

Originally the "uhelper=" was ignored for root, now the umount command
calls /sbin/umount.<uhelper> always.

This change is necessary for things like pam_mount where a device has
to be uninitialized by uhelper rather than directly umounted by
umount(2).

Note that all uhelpers has to be linked with libmount or execute

	umount -fake -i /mountpoint

to remove mountpoint entry from /etc/mtab or /dev/.mount/utab.

Signed-off-by: Karel Zak <kzak@redhat.com>