find_last_path_separator returns NULL if the filename doesn't contain a path separator in it - i.e., it's just a local file. So protect the loop to avoid a segfault

cmr=v1.8.3:reviewer=rolfv
Bring over changes to MXM from pmix branch:

MTL MXM: establish endpoint connection on the first communication when direct_modex used
Merge branch 'master' of https://github.com/open-mpi/ompi-svn-mirror
Some minor cleanups. Main thing is to default to using a blocking fence for now as we don't yet have the non-blocking fence working right.