Language: use UPnP instead of UP&P

Some pages and messages would use UP&P where UPnP should be used. This
replaces all occurences of "UP&P" with the correct "UPnP".

Disclaimer: Yes, this plugin should use l10n in the long run, but this bothers me enough to file a pull request (and it would be nice if the eventual l10n source strings already used correct terminology). UP&P is meaningless, UPnP is the name of the protocol partially implemented by this plugin.