Merge: nitpackage: generate and check manpages

This PR adds two new features to nitpackage:

--check-man: check manpages existence and content for packages and executable modules
--gen-man: create manpages for packages and executable modules

As example I added new manpages (91b9839) to `lib/markdown` and `lib/github`.

Please only consider the 6 last commits as the rest belongs to #2652, #2653, #2656, #2657 and #2662.

Pull-Request: #2663
Reviewed-by: Jean Privat <jean@pryen.org>