Further work on FORTIFY_SOURCE

Hi,

Sponsored by Stormshield and Klara, I've completed an extension of the 
original FORTIFY_SOURCE work that landed a couple weeks ago.  This set 
includes both a test framework to make sure that it's more or less 
working as we expect, along with fortification of many, many more libc 
functions & syscalls.  The relevant reviews are in a stack starting at 
https://reviews.freebsd.org/D45676 and include:

D45676 Prepare some build fixes in advance of more _FORTIFY_SOURCE
D45677 libc: move __ssp_overlap back out into <ssp/ssp.h>
D45678 libc: tests: add testing infrastructure for _FORTIFY_SOURCE
D45679 include: ssp: round out fortification of current set of headers
D45680 include: ssp: fortify poll/ppoll from <poll.h>
D45681 include: ssp: fortify <stdlib.h>
D45682 include: ssp: fortify <wchar.h>
D45683 include: ssp: fortify <sys/random.h>
D45684 include: ssp: fortify <sys/uio.h>
D45685 include: ssp: fortify <sys/select.h>
D45686 include: ssp: fortify <sys/socket.h>

I've pushed a branch to Klara's GitHub as well for anyone interested in 
trying to consume it in that format:

https://github.com/KlaraSystems/freebsd/commits/stormshield/fortify_source/

Review and feedback are welcome- I've been running the full set on my 
laptop and my build machine with no obvious issues

One caveat that we'll need to work through is that the lang/gcc* ports 
include some <ssp/*> headers, so those ports can't be used to build 
anything with _FORTIFY_SOURCE enabled (not compatible with ours).  These 
headers should almost certainly just go away at this point -- the 
improvement in the feature with GCC comes from the builtin 
implementations, not from whatever they've written in these headers.

Thanks,

Kyle Evans