Quit child processes when supervisor crashes (Linux only)

If supervisor crashes, its child processes left running and inherited by supervisor's parent process.  I've added a way to send these processes a stop signal if the parent goes away using prctl(PR_SET_PDEATHSIG, SIGTERM).  It only works on Linux.  On other platforms exceptions are caught and ignored.

I've not run it on anything besides Linux yet.

To test it, start supervisor then send it a kill -9.  The child process receives the SIGTERM as expected.