This patch implements local username normalization using PRECIS
(https://tools.ietf.org/html/rfc7564,
https://tools.ietf.org/html/rfc7613)
It makes chasquid accept local email and authentication regardless of
the case. It covers both userdb and aliases.
Note that non-local usernames remain untouched.
This patch implements the AUTH SMTP command, using per-domain user databases.
Note that we don't really use or check the validation for anything, this is
just implementing the command itself.