Rewrite manpage with mdoc(7)

[Artoria2e5]

mdoc is a different, not quite new, macro package for writing manual pages. Its main point is to write in a more semantic way that serves search systems and HTML output better.

Among other things, mdoc(7) has you:

• use macros instead of manual indents in code samples
• use specialized macros for referring to other man pages (turns into hyperlink in PDF/HTML)

(The table should be rewritten with tbl(7) anyways, man or mdoc.)

[rrthomas]

I would be happy to get a pull request for that.