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.