Fix the GCC build after _FORTIFY_SOURCE import

We haven't exposed gets(3) in a long time, rip out __gets_chk before
it's too late and something builds a gets(3) user with it enabled.

Import _FORTIFY_SOURCE implementation from NetBSD

This is a mostly-unmodified copy of the various *_chk implementations
and headers from NetBSD, without yet modifying system headers to start
actually including them. A future commit will also apply the needed
bits to fix ssp/unistd.h.

Reviewed by: imp, pauamma_gundo.com (both previous versions), kib
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D32306

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Remove the SYMVER build option.

This option was added as a transition aide when symbol versioning was
first added. It was enabled by default in 2007 and is supported even
by the old GPLv2 binutils. Trying to disable it currently fails to
build in libc and at this point it isn't worth fixing the build.

Reported by: Michael Dexter
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D24637

libc: don't build compat functions if building WITHOUT_SYMVER

WITHOUT_SYMVER necessarily implies building a system without symver
backwards compatability.

Sponsored by: The FreeBSD Foundation

Move the stack protector to a new "secure" directory

As part of the code refactoring to support FORTIFY_SOURCE we want
a new subdirectory "secure" to keep the files related to security.
Move the stack protector functions to this new directory.

No functional change.

Differential Review: https://reviews.freebsd.org/D3333