Fix GSMTAP capture restart: recreate UDP socket on start after stop

GsmtapListener.stop() closed the UDP socket permanently, but start()
only set _running=True without recreating it. Second capture session
would silently spin on the dead socket (OSError → None → continue).

- Add _closed flag to GsmtapReceiver, set in close()
- Store bind_port in GsmtapListener for socket recreation
- start() recreates GsmtapReceiver only if previous one was closed
- Uses getattr for backward compat with test fakes

simtrace2_pysniff/server/capture.py

@@ -34,11 +34,14 @@ def gsmtap_msg_type(sub_type):
 
 class GsmtapListener:
     def __init__(self, bind_port=4729):
+        self._bind_port = bind_port
         self._receiver = GsmtapReceiver(bind_port=bind_port)
         self._running = False
         self._dropped = 0
 
     def start(self):
+        if getattr(self._receiver, '_closed', False):
+            self._receiver = GsmtapReceiver(bind_port=self._bind_port)
         self._running = True
         self._dropped = 0