MFC r307656, r307659, r307674-r307675, r307679, r307683

Support for WITHOUT_GNU_DIFF and WITHOUT_GNU_GREP, plus manually
regenerated src.conf.5, which seems to have picked up a couple changes
beyond what was in this MFC.

r307656:
Put each SUBDIR on a separate line for ease of maintenance

Additional patches to this file are in progress, and having each SUBDIR
entry on a separate line makes it easier to change the order in which
the patches are reviewed, tested, and applied.

r307659:
Switch gnu/usr.bin/Makefile to SUBDIR.${MK_*} optional subdir style

r307674:
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

r307675:
Remove trailing whitespace from r307674

r307679:
Build libgnuregex only if necessary for other components

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D8298

r307683:
Correct typo in r307679: the variable is MK_GNU_GREP_COMPAT