Add manxref, a tool for analyzing man-page crossreferences across a
whole system's worth of man pages.

Pursuant to PR 9627, and a much simpler script therein, where it was
suggested that man page crossreferences mostly ought to be
bidirectional.

This does not appear to be the case. There are a lot of cases where a
reference in one direction does not imply a reference back; for
example, make(1) refers to chdir(2) for good reasons, but there's no
sensible reason for chdir(2) to refer to make(1). Likewise, lots of
driver pages refer to bus pages for buses they attach to, but the
reverse links aren't necessarily useful as there are a lot of them.

Note: this program is not related to the Isle of Man.

(dholland)