Remove $FreeBSD$: one-line nroff pattern

Remove /^\.\\"\s*\$FreeBSD\$$\n/

Rename META_MODE option to DIRDEPS_BUILD

This allows META_FILES option to be renamed META_MODE.
Also add META_COOKIE_TOUCH for use in targets that can benefit
from a cookie when in meta mode.

Differential Revision: https://reviews.freebsd.org/D4153
Reviewed by: bdrewery

Fix generation of src.conf.5

Since makeman turns all options on, we need to guard somethings from
make(showconfig)