Otherwise it could be inherited by other jwack jobs started from the same
parent process, resulting in some very-hard-to-debug race conditions, let me
tell you.
This avoids a second fork altogether, which apparently doesn't matter all
that much (5.3s -> 5.15s). However, simply not importing the subprocess
module reduces us further to 4.6s.
And 'ps axf' now looks prettier because we have fewer stupid intermediate
processes.
instead of inside the fork.
Still doesn't seem to affect runtime. Good.
One nice side effect is jwack.py no longer needs to know anything about our
locks.
We can also avoid forking altogether if should_build() returns false. This
doesn't seem to result in any noticeable speedup, but it's cleaner at least.
Previously, the default was to *always* keep going, which is actually not
usually what you want. Now we actually exit correctly after an error. Of
course you still might have multiple errors before existing if you were
building in parallel.
Get rid of the "locked..." and "...unlocked!" messages by default, since
they're not usually interesting. But add a new option to bring them back in
case we end up with trouble debugging the locking stuff. (I don't really
100% trust it yet, although I haven't had a problem for a while now.)
