feature/relayserver: init server at config time instead of request time (#17484)

The lazy init led to confusion and a belief that was something was
wrong. It's reasonable to expect the daemon to listen on the port at the
time it's configured.

Updates tailscale/corp#33094

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/udprelay/server.go

@@ -536,6 +536,7 @@ func (s *Server) listenOn(port int) error {
 			s.uc6 = bc
 			s.uc6Port = uint16(portUint)
 		}
+		s.logf("listening on %s:%d", network, portUint)
 	}
 	return nil
 }