Null Backend Interface for OpenLDAP

The Null backend is surely the most useful part of slapd:
- Searches return success but no entries.
- Compares return compareFalse.
- Updates return success (unless readonly is on) but do nothing.
- Binds fail unless the database option "bind on" is given.
  The "bind" option is "off" by default.
Inspired by the /dev/null device.

slapd.conf example:
  database null
  suffix   "cn=Nothing"
  bind     on