Programs that manage a set of workers
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  Programs that start with 'p' are multi-process workers,
  programs that start with 't' are multi-thread workers.


* pworkers1.c:  signal-based manager of 'n' worker processes (sigsuspend)
* pworkers2.c:  signal-based manager of 'n' worker processes (sigwaitinfo)
                Using sigwaitinfo the main loop accepts the signal directly
                so no signal handler is involved. This is a nice approach.
* tworkers1.c:  main thread keeps a queue serviced by 'n' worker threads 
* tworkers2.c:  main thread keeps a queue serviced by 'n' worker threads.
		When the main thread receives a signal, it cancels the worker
                threads and 'joins' (collects) them. There is a trick needed
                in this program: see the use of pthread_cleanup_push/pop.
                Without their use, the mutex locked (when pthread_cond_wait
                is canceled) would remain locked and prevent the other threads
                from being able to cancel. By installing the pop handler to
                release the mutex, it's released correctly in both the cancel 
                scenario or in normal operation. (c.f. TLPI p.678).
* tworkers3.c:  Variant of tworkers2; uses a 'shutdown' variable with a
		condition broadcast instead of pthread_cancel. Accomplishes
		the same result as tworkers2 with less complexity.


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Also included:

 quitafter.c:   Runs a sub-program, forcing it to quit after x seconds, e.g.:

                    quitafter 10s /bin/sleep 50