This is what SASL LOGIN should look like (adapted from a patch

This is what SASL LOGIN should look like (adapted from a patch
sent by Aleksandr Pokotilo). This doesn't work, but making it
work involves changes to how SaslMechanism is called, and I'll
do that separately, along with removing the many (4!) copies of
that fairly complex code that we have right now.

p4raw-id: //oryx/src/main@54150