zeroconf: suppress traceback during shutdown

If the read thred is in select when the main thread is in close, the main
thread may close the socket between select and read, generating a noisy
traceback. This can be ignored if the shutdown flag is set.

hgext/zeroconf/Zeroconf.py

@@ -863,7 +863,8 b' class Engine(threading.Thread):'
 						try:
 							self.readers[socket].handle_read()
 						except:
-							traceback.print_exc()
+							if not globals()['_GLOBAL_DONE']:
+								traceback.print_exc()
 				except:
 					pass