Add a build knob for _FORTIFY_SOURCE

Add a build knob for _FORTIFY_SOURCE

In the future, we will Default to _FORTIFY_SOURCE=2 if SSP is enabled,
otherwise default to _FORTIFY_SOURCE=0.  For now we default it to 0
unconditionally to ease bisect across older versions without the new
symbols, and we'll put out a call for testing.

include/*.h include their ssp/*.h equivalents as needed based on the
knob. Programs and users are allowed to override FORTIFY_SOURCE in their
Makefiles or src.conf/make.conf to force it off.

Reviewed by:	des, markj
Relnotes:	yes
Sponsored by:	Stormshield
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D32308