Basic reporting of .Xrs to manual pages that don't exist

Basic reporting of .Xrs to manual pages that don't exist
in the base system, inspired by mdoclint(1).

We are able to do this because (1) the -mdoc parser, the -Tlint validator,
and the man(1) manual page lookup code are all in the same program
and (2) the mandoc.db(5) database format allows fast lookup.

Feedback from, previous versions tested by, and OK jmc@.

A few features will be added to this in the tree, step by step.