packages: Add a mandoc package

Move mandoc to its own package so users can install it independently of
-utilities. Put the package in the minimal set, since we also ship
manpages in minimal and "man" is a basic Unix utility.

Add a pkg-triggers(5) hook to run makewhatis when new manpages are
installed, so that apropos(1) works. This depends on a new pkg(8)
feature expected to be in in the upcoming 2.3.2 release; in the mean
time, this is a no-op (i.e., having an older pkg doesn't break anything,
it just won't run the trigger).

MFC after: 3 seconds
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D52564