atoi() was getting redundant, and unfortunately we can't easily load
helpers.py in some places where we'd want to, because it depends on vars.py.
So move it to its own module.
We'll have to stop using nonblocking reads, unfortunately. But this seems
to work better than nothing. There's still a race condition that could
theoretically make GNU make angry, unfortunately, since we briefly set the
socket to nonblocking.
But it seems to be pretty unsolvable in the current form; the problem is
that when you're nesting one jwack inside the other and the jobserver is GNU
make, there's no way to tell the parent jwack not to use up a token. Thus,
if you nest too deeply, it just deadlocks.
So this approach isn't really going to work the way it is.
