lib/nat, lib/upnp: IPv6 UPnP support (#9010)

This pull request allows syncthing to request an IPv6
[pinhole](https://en.wikipedia.org/wiki/Firewall_pinhole), addressing
issue #7406. This helps users who prefer to use IPv6 for hosting their
services or are forced to do so because of
[CGNAT](https://en.wikipedia.org/wiki/Carrier-grade_NAT). Otherwise,
such users would have to configure their firewall manually to allow
syncthing traffic to pass through while IPv4 users can use UPnP to take
care of network configuration already.

### Testing

I have tested this in a virtual machine setup with miniupnpd running on
the virtualized router. It successfully added an IPv6 pinhole when used
with IPv6 only, an IPv4 port mapping when used with IPv4 only and both
when dual-stack (IPv4 and IPv6) is used.

Automated tests could be added for SOAP responses from the router but
automatically testing this with a real network is likely infeasible.

### Documentation

https://docs.syncthing.net/users/firewall.html could be updated to
mention the fact that UPnP now works with IPv6, although this change is
more "behind the scenes".

---------

Co-authored-by: Simon Frei <freisim93@gmail.com>
Co-authored-by: bt90 <btom1990@googlemail.com>
Co-authored-by: André Colomb <github.com@andre.colomb.de>

cmd/strelaysrv/main.go

@@ -194,7 +194,15 @@ func main() {
 		cfg.Options.NATTimeoutS = natTimeout
 	})
 	natSvc := nat.NewService(id, wrapper)
-	mapping := mapping{natSvc.NewMapping(nat.TCP, addr.IP, addr.Port)}
+	var ipVersion nat.IPVersion
+	if strings.HasSuffix(proto, "4") {
+		ipVersion = nat.IPv4Only
+	} else if strings.HasSuffix(proto, "6") {
+		ipVersion = nat.IPv6Only
+	} else {
+		ipVersion = nat.IPvAny
+	}
+	mapping := mapping{natSvc.NewMapping(nat.TCP, ipVersion, addr.IP, addr.Port)}
 
 	if natEnabled {
 		ctx, cancel := context.WithCancel(context.Background())