PSM MTL: Don't connect procs already connected

PSM has issues when trying calling psm_ep_connect() more than once for a
specific peer.  Use the psm_ep_connect mask argument to avoid connecting
to processes that are already connected.

Verified by myself and @adrianreber.

Fixes OMPI ticket #268.
(cherry picked from commit open-mpi/ompi@b97cda7fd92cc1cafef46a59758d0788f1b5917f)