Per-process PS_STRINGS, USRSTACK, shared_obj_base, and sigcode_base

[Shawn Webb]

Hey All,

I've updated this patch to the latest HEAD. This patch is a preparatory patch
for ASLR. It makes it so that PS_STRINGS, USRSTACK, the VDSO, and the signal
trampolining code pointers are all stored per-process. Going forward, with
HardenedBSD's ASLR patch, these values will be modified to introduce
randomness/entropy. The goal with this patch is to allow randomization of the
stack base address and the VDSO.

You can find the patch here: https://reviews.freebsd.org/D3565

Thanks,