in list_iterator_destroy(). This caused a race condition
   where iterator could be pointing to new list by the time
   list_mutex_unlock() was called.

   (HAVE_PTHREAD implies HAVE_PTHREAD_H)
 o fix hostlist_*_string() functions to return -1 on trucation

pthread_create failure.

 o set WITH_PTHREADS in addition to HAVE_PTHREAD in config.h

Iterators now given their own magic cookie in list.c (former shared
cookie value with list entity).

insure the hash table does not overflow.

   point. This fixes a race condition in srun.

Added semaphores for some job_step functions that were lacking them.

 o do not block all signals

 o call log_reinit() after fork() in pthreaded app to reinit log mutex,
   but keep same log options.

 o inserted comments in some areas of confusion or possible problems.