builder.py: further refactoring to run more stuff in the parent process

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.

jwack.py

@@ -182,9 +182,8 @@ class Job:
         return 'Job(%s,%d)' % (self.name, self.pid)
 
             
-def start_job(reason, lock, jobfunc, donefunc):
+def start_job(reason, jobfunc, donefunc):
     global _mytokens
-    assert(lock.owned)
     assert(_mytokens <= 1)
     get_token(reason)
     assert(_mytokens >= 1)
@@ -203,12 +202,9 @@ def start_job(reason, lock, jobfunc, donefunc):
             except Exception:
                 import traceback
                 traceback.print_exc()
-            lock.unlock()
         finally:
             _debug('exit: %d\n' % rv)
             os._exit(rv)
-    # else we're the parent process
-    lock.owned = False # child owns it now
     os.close(w)
     pd = Job(reason, pid, donefunc)
     _waitfds[r] = pd