Change SLAPD shutdown to do a cond wait on active threads instead
of a busy wait and allow the pthread_yield within the loop to be removed. This was the only pthread_yield which was necessary when usng non-preemptive threads. As such, the configure.in sched_yield/pthread_yield missing error can be removed from configure.in. If explicit yield function is missing, just provide a no-op replacement. Moved declaration of various slapd globals from main.c to init.c so they can be shared with ldbm tools.