Make ldap_int_thread_userctx_t.ltu_key[] a proper NULL-terminated array.

(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).