bin/xbps-uhelper: add longopts, manpage

[classabbyamp]

• bin/xbps-uhelper: add longopts for -CdrV
• bin/xbps-uhelper: add manpage
• data/_xbps: add xbps-uhelper getname, getversion to completions

I started (and mostly finished) this manpage before looking at the open PRs and seeing #305, and seeing as that PR has sat for a year and a half now, I'm submitting my alternative take on the uhelper manpage.

It's mostly the same, with some minor formatting differences, and a couple additional sections.

I did not include xbps-uhelper fetch and xbps-uhelper digest in the manpage because they are replaced by xbps-fetch and xbps-digest (https://github.com/void-linux/xbps/blob/master/NEWS#L357-L359).

closes #305

[classabbyamp]

interestingly, the zsh completions for uhelper already had the longopts in them

[Duncaen]

Not sure about adding xbps-uhelper to see also in other pages, its not really relevant for most usecases.

[classabbyamp]

Not sure about adding xbps-uhelper to see also in other pages, its not really relevant for most usecases.

fair enough