Merge some/most of schwarze@'s OpenBSD changes into mandoc: many more

Merge some/most of schwarze@'s OpenBSD changes into mandoc: many more
warnings about grokking manpages in their respective directories.
DO NOT, however, import his temporary-file routines (I don't plan on
staying with a recno/btree split) nor the realpath() routines, which
destroy relative path-ness.

Also pull in the lorder bits.

There are some changes I started to make then stopped relating to
reporting errors in the correct directories.  I'll clean this up in
subsequent commits.

This puts us more or less on parity with OpenBSD.