OSX tempdir too long for sun_path

[hpcraink]

Per the discussion in the pull request in pmix/master#71 on Mac OSX, the temporary directory can get too long for the socket's sun_path (which is on LINUX 108 chars, on Mac OSX 103 chars), making Open MPI fail with cryptic: "bind() failed"...

Ralph suggested moving the pull request to OMPI.

So, now fail gracefully and have OMPI nicely complain to the user:

USER@SUPER-MacBook-Pro:~/C/MPI_TESTS> ./mpi_wtick
[SUPER-MacBook-Pro.local:28645] [[46320,0],0] bind() failed on error Address already in use (48)
--------------------------------------------------------------------------
The call to pmix_init_server() failed. This may be due to your
system's restriction for Unix's socket's path-length.

orte_proc_session_dir: /var/folders/30/c1t3_6m546n62qrckq5fs_wc0000gp/T/openmpi-sessions-502@USER-SUPER-MacBook-Pro_0/46320/0/0

Please try to set TMPDIR to something short (like /tmp) or change
Your computer's name (see uname -n).