share/mk: Add MIT KRB5 build knob

This is the first in a series of commits to replace Heimdal with
MIT KRB5. This first commit adds the WITH_MITKRB5/MK_MITKRB5 knob
to src.opts.mk and its corresponding documentation in
tools/build/options. The default is off.

This change of and by itself is of no consequence as MIT KRB5 has
yet to be imported in to HEAD. But it does insulate the build from
inremental change until the last patch is committed when WITH_MITKRB5
will build MIT KRB5 1.21.3 instead of Heimdal.

The affords us the opportunity to review smaller commits.

This is the first of many commits.

Reviewed by: imp, jhb, brooks, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50684