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.