terminate subprocesses on shutdown (e.g. on shutdown via dbus or SIGTERM)

* fixes program hang on shutdown
* improves consistency - terminating Mailnag via CTRL+C always did this

Mailnag/plugins/userscriptplugin.py

@@ -25,7 +25,7 @@ import os
 from gi.repository import Gtk
 from common.plugins import Plugin, HookTypes
 from common.i18n import _
-from common.utils import start_subprocess
+from common.subproc import start_subprocess
 
 plugin_defaults = { 'script_file' : '' }