cmd/tailscale/cli: allow remote target as service destination (#17607)

This commit enables user to set service backend to remote destinations, that can be a partial
URL or a full URL. The commit also prevents user to set remote destinations on linux system
when socket mark is not working. For user on any version of mac extension they can't serve a
service either. The socket mark usability is determined by a new local api.

Fixes tailscale/corp#24783

Signed-off-by: KevinLiang10 <37811973+KevinLiang10@users.noreply.github.com>

net/netns/netns_default.go

@@ -20,3 +20,7 @@ func control(logger.Logf, *netmon.Monitor) func(network, address string, c sysca
 func controlC(network, address string, c syscall.RawConn) error {
 	return nil
 }
+
+func UseSocketMark() bool {
+	return false
+}

net/netns/netns_dw.go

@@ -25,3 +25,7 @@ func parseAddress(address string) (addr netip.Addr, err error) {
 
 	return netip.ParseAddr(host)
 }
+
+func UseSocketMark() bool {
+	return false
+}