Update for v1.5.1 release.

Fix sasl for DIGEST-MD5, PLAIN, and LOGIN mechanisms, which I don't think ever actually worked properly. Add a sasluser argument if a specific identity is required for the backend, which if unsupplied, tries to guess if a binddn is present. (Uwe's previous commit fixed EXTERNAL and GSSAPI, which did work, but randomly failed due to hash ordering.) Minor style cleanups, remove duplicate/unecessary logic for anonymous binds.

Don't provide a password for sasl authentication. When a sasl parameter is given (and used) the password parameter is not used by Net::LDAP. If indeed a password is required it has to be passed in the Authen::SASL object, not as parameter to bind. So drop the password parameter which stops trying (and failing) to use password authentication sometimes if in Net/LDAP.pm the iteration over %ptype hits its 'password' member before the 'sasl' member.