text-based mount command: make po_rightmost() work for N options

Sometimes we need to choose the rightmost option among multiple
different mount options.  For example, we want to find the rightmost
of "proto," "tcp," and "udp".  Or, the rightmost of "vers," "nfsvers,"
"v2," and "v3".

Update po_rightmost() to choose among N options instead of just two.

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