jwack: start waitfds around fd#50.
jwack: start waitfds around fd#50.

That makes it a little easier to tell, in a strace, what the process is
waiting on.  If it's 100/101, then it's waiting on a token; 50+ means waiting
on a subtask.

Also, we weren't closing the read side of subtask fds on exec.  This didn't
cause any problems, but did result in a wasted fd in subprocesses.

dinkelk/redo-python,pombredanne/redo,pombredanne/redo,rowhit/redo,pombredanne/redo,pombredanne/redo,apenwarr/redo,rowhit/redo,apenwarr/redo,apenwarr/redo,rowhit/redo,dinkelk/redo-python,apenwarr/redo,rowhit/redo,dinkelk/redo-python,dinkelk/redo-python