libmount: (subdir) support unshared sessions too

libmount: (subdir) support unshared sessions too

* enter original namespace for the final move_mount(), it means that
  the original unshare() (to create the temporary mount) will be
  ineffective for this final step. God bless FD based APIs (setns()
  and open_tree())! ;-)

* improve cleanup code (call umount only once)

* remember private namespace

* update note in man page

Addresses: https://github.com/util-linux/util-linux/pull/2198
Signed-off-by: Karel Zak <kzak@redhat.com>