Catch the standard Unix SIGTERM kill signal and do a graceful shutdown of PyBitmessage.

We already do this for the SIGINT kill signal. The change allows us to do a clean
shutdown of PyBitmessage when its process has been separated from the terminal.

src/bitmessagemain.py

@@ -172,6 +172,7 @@ class Main:
             curses = True
 
         signal.signal(signal.SIGINT, helper_generic.signal_handler)
+        signal.signal(signal.SIGTERM, helper_generic.signal_handler)
         # signal.signal(signal.SIGINT, signal.SIG_DFL)
 
         helper_bootstrap.knownNodes()