check for NULL dn in URL; set error code before seding extended response (partially addresses ITS#4968)

The Written Word and Stanford University.

Corrected rev 1.67 cvs comment: ltu_key[] is NULL-terminated _when not full_.

threads per pool, even when ltp_max_count <= 0 ("unlimited").
Keep ltp_max_count in range [0, LDAP_MAXTHR].

(setkey/getkey expected that, but purgekey could set a NULL in the middle.)
Added some checks for input key!=NULL.

API changes, need review - I'm not sure what's indented here:
- setkey(data=NULL, kfree!=NULL) searched as if intended to reset
  the key, but updated by setting the key.  Now always updates.
- setkey(key=<not found>, data=NULL) could return either success or
  failure.  Now succeeds iff (data == NULL && kfree == NULL).

"deleted item" mark.  Add DELETED_THREAD_CTX.
Also improve the hash function a bit, and make the hash unsigned.

ldap_pvt_thread_pool_purgekey() which may only be called during pauses.

Thus, also wait for pauses to finish before accessing thread_keys in
ldap_int_thread_pool_wrapper().  This may prevent pending tasks from
being started when a pause had been requested, which seems to have been
possible.  If that was a feature, we can split ltp_pause==1 in 2 states:
in pause (causes wait), and pause requested.

Also move 'thread_keys[].id = <thread id>' from pool_submit to
pool_wrapper.  Until pool_wrapper set the ctx as well, thread context
lookup would just return NULL anyway.