upnp discovery broken on multi-homed machines

UPnP discovery is broken on machines connected to multiple networks (this might also be the cause of #5208 (closed)). The problem is within libupnp: As documented for example here: http://libupnp4.sourcearchive.com/documentation/1.8.0~svn20100507-1.1/group__UPnPAPI_ga387e7c62de9baeef328bd59ba81f99ae.html#ga387e7c62de9baeef328bd59ba81f99ae, UpnpInit() does only initialize UPnP for a single network adapter. Appearantly, it's not possible to use multiple adapters. This has to be fixed by the libupnp maintainer(s), I think. Additionally, depending on which version of libupnp is used, UpnpInit() should be replaced by UpnpInit2(), which also supports IPv6.