Resolve the MTT timeout problem. This appears to have largely been caused by missing sigchld notifications, thus causing the daemons to believe that not all procs had exited. Let comm failure also serve as notification of process termination, and add appropriate flags/attributes to avoid multiple reporting of proc termination.

This won't transition cleanly to the 1.8 series, and may represent too much change, so we'll have to (a) evaluate whether or not to bring it over (once it demonstrates that it does indeed solve the problem), and (b) develop a custom patch for that purpose.

Refs #4717