Rather than calling check() on backedges, inline code to check the
flags.  Also, check flags on recursion and add some comments explaining
why we do that.  Net result: 40% speedup on my version of mandelbrot,
at least on one machine.
Use the same trigger mechanism for parallel aborts as for everything else
Add a configure flag to enable manual thread control,
allowing it to be used in opt builds as well as debug builds.
Correct lastLockedObject to ObjectSet
Fix incorrect and racy swapping of mainThread.ionTop in LockedJSContext
Modify the parjs benchmark script to only us to run only seq
or only parallel
Merge branch 'master' into pjs-stop-the-world-gc

Resolved conflicts:
	js/src/vm/ForkJoin.cpp