pbzip2 v1 - add ability to dynamically change thread use based on load

[Jeff Gilchrist]

A wishlist feature from: Greg Lindahl [ lindahl [at] pbm <dot> com ]

" if I have -l on, it would be nice for the threads (or perhaps just the initial thread) to watch the load average, and if it climbs during the compression run (which might last 10+ of minutes), use fewer threads. If the max processors aren't capped, -l would now protect against starting several pbzip2s at once, or against other activities suddenly making the system busier than it was when pbzip2 started."