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/gsmtap.py

@@ -79,6 +79,7 @@ class GsmtapReceiver:
         self._sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
         self._sock.bind((bind_ip, bind_port))
         self._sock.settimeout(1.0)
+        self._closed = False
 
     def read_packet(self):
         """Block until a GSMTAP-SIM packet arrives.
@@ -109,3 +110,4 @@ class GsmtapReceiver:
 
     def close(self):
         self._sock.close()
+        self._closed = True