Do a little better job of cleaning up the session directory left by mpirun by ensuring we delete the event associated with debugger attachment and unlinking the pipe used for that purpose. Also, we no longer leave "abort" files around, so remove that check when deleting session directory trees

cmr=v1.7.5:reviewer=jsquyres:subject=cleanup session directories better
Improve the script to ignore executables and Mac-specific files of no interest