RESQUE_MEM_THRESHOLD restarts child after too much memory growth

I've been running this in production for the last week, and it's been working out very nicely for us.  There's no good reason for a memory leak in one type of job to bring down an entire system with thrashing.  It uses the `/proc` filesystem under Linux for (*much*) faster and leaner measurement, but has also been tested under Mac OS X (using slower fork/exec `ps`).