xclip.1: rewrite in the mdoc(7) format

[guijan]

As suggested in #96. The new manual isn't an exact copy, most of it is rewritten.
My name is in the copyright because I started from scratch and wrote most of the text (though it is based off the old manual), I'm not sure how this copyright stuff is supposed to work.
I've also credited myself among the authors, stating clearly that my authorship is limited to this manual, if that is not considered too scandalous.

This manual also includes the example found in #114.

[guijan]

Poking @hackerb9

[robs-nice99]

Hello.
I wanted to add some information to the man page. But I don't know some details... Maybe you can help?
#137

[guijan]

Rebased, I didn't notice the conflict 10 months ago.

Hello. I wanted to add some information to the man page. But I don't know some details... Maybe you can help? #137

Sure, but this PR rewrites the original man manual in mdoc. I need to know if this one will be merged first to avoid wasting effort. It has been a year but it seems to be because xclip development is slow rather than a rejection.