internal/netlink: expose multicast groups

This also gets rid of the cached pid value for port since that prevents multiple sockets from being open at once.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/netlink/rtnl.go

@@ -11,7 +11,7 @@ type RouteConn struct{ *conn }
 
 // DialRoute returns the address of a newly connected [RouteConn].
 func DialRoute() (*RouteConn, error) {
-	c, err := dial(syscall.NETLINK_ROUTE)
+	c, err := dial(syscall.NETLINK_ROUTE, 0)
 	if err != nil {
 		return nil, err
 	}