Add support for MAP_INHERIT_ZERO.

This provides a way for a process to designate pages in its address
space that should be replaced by fresh, zero-initialized anonymous
memory in forked child processes, rather than being copied or shared.

ok jmc, kettenis, tedu, deraadt; positive feedback from many more
Add timingsafe_memcmp().

ok deraadt, jmc, tedu
crank major