gmoccapy: exit on SIGTERM received during startup

#4115 installs the signal handler just before Gtk.main(), so a SIGTERM
arriving during construction is still handled by the excepthook via
KeyboardInterrupt. With no main loop running, Gtk.main_quit() asserts
and the interrupt is swallowed at the GTK C boundary, so the process
runs on until SIGKILL. Force the exit when no loop is active.

Author: grandixximo

src/emc/usr_intf/gmoccapy/gmoccapy.py

@@ -59,10 +59,13 @@ def excepthook(exc_type, exc_obj, exc_tb):
     # caller escalates to SIGKILL.
     if issubclass(exc_type, KeyboardInterrupt):
         LOG.info("gmoccapy interrupted (signal), shutting down")
-        try:
+        if Gtk.main_level() > 0:
             Gtk.main_quit()
-        except Exception:
-            pass
+        else:
+            # Signalled before the main loop starts (e.g. during startup).
+            # The interrupt was swallowed at the GTK C boundary, so returning
+            # here just resumes startup; force the exit.
+            os._exit(0)
         return
     try:
         w = app.widgets.window1