Add test listener no-thread_create experiment (behind ifdef).

I moved slap_shutdown after the waitforkids.  I also wait for each process to complete before returning from fork_child since I had two lockups on my NT box.

Move cache_return_entry_w() out of ldbm_back_add when the add is successful.
This change breaks back-bdb2, it needs equvalent changes of those in back-ldbm.