Prepare the system for _FORTIFY_SOURCE

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