LP1204123 SIGTERM graceful shutdown (C)

LP1204123 SIGTERM graceful shutdown (C)

When a TERM signal is received, the listener parent process will wait
for all child processes to complete any active tasks before killing the
child processes and shutting down.

Receiving a SIGINT or SIGQUIT results in a non-graceful shutdown.

In all cases, the system first un-registers with all routers.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>