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.
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