Connect to the LDAP backend with SASL credentials.

Connect to the LDAP backend with SASL credentials.

This reworks our LDAP backend code to move from anonymous access to a
shared-secret SASL-protected connection.  (SASL selects NTLM or
DIGEST-MD5 on my system).

To get this working, we must pre-populate the LDAP backend with a DN
to store ths SASL secret on, and we use back-ldif for this.

This gives us a reasonable basis to deploy a replicated OpenLDAP
backend solution.

Andrew Bartlett
(This used to be commit cd0745253c4a9ec59a035e830e54d74a05b71aaa)