issues (at least):
1) sbrk is defined as returning char *, which violates XPG
2) I'm not clear on what happens if you try to sbrk() on a negative
incr that is less than the page size. I'm guessing "nothing", but we
should document this behavior.
3) XPG says some interesting things about whether new pages are
guaranteed zeroed. We say nothing about this. We should document our
behavior.
4) It isn't clear if *we* guarantee that malloc and sbrk can be mixed
in our API (I've documented that it isn't universally portable.)
We really should clean these up, too.
eeebdec53e