man: create man page with asciidoctor instead of help2man

[cgzones]

Support building man pages for cross builds.

Include more information, like 'Exit status' and 'Check IDs'.

[dburgener]

I don't like that this duplicates the contents from the README. I certainly am all for better man pages, but I'd prefer to have a single source of truth for documentation. It's already bad enough that we duplicate the arguments in the code for --help and also in the README. If we're going to migrate to a better documentation generator, I'd like it to reduce documentation duplication, and this seems to increase it.