Add knobs to make GNU diff and GNU grep optional

This is added to facilitate experiments building FreeBSD without
copyleft software.

If WITHOUT_GNU_DIFF is set no /usr/bin/diff or /usr/bin/diff3 will
be built.

If WITHOUT_GNU_GREP is set then BSD grep will be installed as
/usr/bin/bsdgrep or /usr/bin/grep, depending on the WITH_BSD_GREP
knob.

Reviewed by: brooks (earlier)
Sponsored by: The FreeBSD Foundation
Differential Revision: Differential Revision: https://reviews.freebsd.org/D8288