Allow users to define UPnP rootDesc

[NarkyAbyss]

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Feature Request

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the miniupnp library fails to scan for my openwrt router: No valid UPNP Internet Gateway Device found.

however, if I manually specify the rootDesc URL it will recognize and interface correctly
Example:

upnpc -u http://192.168.1.1:5000/rootDesc.xml -s
Found valid IGD : http://192.168.1.1:5000/ctl/IPConn

I'm no experienced with this library but can we just pass a url to the rootdescurl variable?

[ReenigneArcher]

I assume you can. I'll consider a PR for this.