This patch implements an Authenticator type, which connections use to
do authentication and user existence checks.
It simplifies the abstractions (the server doesn't need to know about
userdb, or keep track of domain-userdb maps), and lays the foundation
for other types of authentication backends which will come in later
patches.
We should ignore the domains' case, and treat them uniformly, specially when it
comes to local domains.
This patch extends the existing normalization (IDNA, keeping domains as
UTF8 internally) to include case conversion and NFC form for
consistency.
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.
