r/w locks and thread pools.  Hide internal structures (using
pthread'ish technics).  Place common code in threads.c.  Move
no-thread code to thr_stub.c.  Move thread pool code to tpool.c.
Removed setconcurrency call from initializer, added 'concurrency'
directive to slapd.  Tested code under pthreads, pth, and no-threads.

other minor changes.

 ldap_pvt_thread_pool_backload() as needed.  All tests run OK on pthre
ads (linux), cthreads, NT threads, and Pth.

Copyright 2000 Stig Venaas, UNINETT
All rights reserved.

Redistribution and use in source and binary forms are permitted
without restriction or fee of any kind as long as this notice is
preserved. The name of UNINETT may not be used to endorse or promote
products derived from this software without specific prior written
permission. This software is provided ``as is'' without express or
implied warranty.

copy of the code formerly in mail500, to properly track changes.  An
exception is mail500.m4 that has been committed as maildap.m4 directly
to avoid breaking history twice.

to put it in line with current practices.  Most likely I got them wrong,
so maight not even compile right now.