net/{batching,packet},wgengine/magicsock: export batchingConn (#16848)

For eventual use by net/udprelay.Server.

Updates tailscale/corp#31164

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

wgengine/magicsock/magicsock_default.go

@@ -21,7 +21,3 @@ func (c *Conn) listenRawDisco(family string) (io.Closer, error) {
 func trySetSocketBuffer(pconn nettype.PacketConn, logf logger.Logf) {
 	portableTrySetSocketBuffer(pconn, logf)
 }
-
-const (
-	controlMessageSize = 0
-)