SASL fixes - parsing and the DH-AES mechanism

1. Fix up parsing of parameters for the AUTHENTICATE command a bit--since everything for SASL is base64-encoded, there's no need to check more than one parameter.

2. Implement the DH-AES mechanism in DB services.  The changes have been tested and are known to work with clients supporting either or both mechanisms.  (i.e. nothing is broken)
I previously described my want and rationale to implement the DH-AES mechanism in the mtxircd pull request (#151).