Add man pages for libXpresent

Merged Alan Coopersmith requested to merge alanc/libxpresent:man into master

Fixes: #1 (closed)

Almost entirely copied from presentproto.txt and reformatted to man page markup, with some minor clarifications.

Suffers from the same problems as presentproto, such as not explaining what msc and ust are, but at least provides a much more solid starting base to improve from.

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