Use prctl() to have the kernel SIGTERM background processes on exit.

Use prctl() to have the kernel SIGTERM background processes on exit.

This uses a Linux-ism to attempt to always clean up background processes
if possible.  Note that it is not a catch-all, in that executables with
suid/sgid or elevated capabilities will have the prctl() attribute
stripped as part of the execve().

Resolves ticket 15471.