wgengine/{magicsock,userspace,router}: move portupdates to the eventbus (#17423)

Also pull out interface method only needed in Linux.

Instead of having userspace do the call into the router, just let the
router pick up the change itself.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

wgengine/router/osrouter/router_openbsd.go

@@ -238,13 +238,6 @@ func (r *openbsdRouter) Set(cfg *router.Config) error {
 	return errq
 }
 
-// UpdateMagicsockPort implements the Router interface. This implementation
-// does nothing and returns nil because this router does not currently need
-// to know what the magicsock UDP port is.
-func (r *openbsdRouter) UpdateMagicsockPort(_ uint16, _ string) error {
-	return nil
-}
-
 func (r *openbsdRouter) Close() error {
 	cleanUp(r.logf, r.tunname)
 	return nil