plugins: Log plugin names as we ask them to stop, and when we're done with them all

This should point the finger at any plugin(s) that aren't stopping
properly.  Well, at least those that are hanging *in* their
`plugin_stop()`.

plug.py

@@ -251,10 +251,14 @@ def notify_stop():
         plugin_stop = plugin._get_func('plugin_stop')
         if plugin_stop:
             try:
+                logger.info(f'Asking plugin "{plugin.name}" to stop...')
                 newerror = plugin_stop()
                 error = error or newerror
             except Exception as e:
                 logger.exception(f'Plugin "{plugin.name}" failed')
+
+    logger.info('Done')
+
     return error