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