wgengine/magicsock: remember fixed port number preference
So LinkChange events rebind to the same port when possible.
@@ -140,6 +140,7 @@ func Listen(opts Options) (*Conn, error) {
epUpdateCtx, epUpdateCancel := context.WithCancel(context.Background())
c := &Conn{
pconn: new(RebindingUDPConn),
+ pconnPort: opts.Port,
donec: make(chan struct{}),
stunServers: append([]string{}, opts.STUN...),
startEpUpdate: make(chan struct{}, 1),
