Add optional second user, then tv_merge the results

[Sparticuz]

This PR adds an optional second user, which, if filled out, will run zap2xml.pl a second time, retrieving the xml file for the second user. This will then be sorted using tv_sort, then merged using tv_merge, which outputs to XMLTV_FILENAME.

This is a non-breaking change. If USERNAME2 is not filled out, it will continue to work as is.

The only change to existing code was adding in xmltv from the Alpine repos.