Improved LDAP authenticator

The LDAP authenticator in HEAD has been improved (and documented) upon a
lot to allow integration with other or more complex LDAP directories,
and it would would be good if someone else could test that it works on
their system.

I have tested it using OpenLDAP 1.2.9 against a small OpenLDAP 1.2.9
server and a Microsoft Active Directory server in all modes I could
think of and it seems to work here..

I plan on announcing the improved version in a week or so after some
further testing, and maybe to commit it to the 2.4 branch as well
(opinions please).

--
Henrik