man: Fix handling of definition lists in man pages

Existing man processing was totally broken, since remark doesn't
natively support definition lists.

Unfortunately, remark-man doesn't natively support definition lists
either, so we have to fake things by altering the syntax tree created
by remark parser and convert the various definition list entries.

This still isn't working 100% (multiple nested definition lists cause some
indention issues), but it is much improved over previous output.