Prepare the system for _FORTIFY_SOURCE

Notably:

• libc needs to #undef some of the macros from ssp/* for underlying implementations
• ssp/* wants a __RENAME() macro (snatched more or less from NetBSD)

There's some extra hinkiness included for read(), since libc spells it
as "_read" while the rest of the world spells it "read."

Reviewed by: imp, ngie
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D32307