Some deployments already have users that authenticate without a domain.
Today, we refuse to even consider those, and reject them at parsing time.
However, it is a use-case worth supporting, at least with some
restrictions that make the complexity manageable.
This patch changes the auth package to support authenticating users
without an "@domain" part.
Those requests will always be directly passed on to the fallback
authenticator, if available.
The dovecot fallback authenticator can already handle this case just fine.
Dovecot has options for changing the formatting of usernames; for
example, dropping the domain part, or replacing characters.
chasquid's implementation, however, fails to handle this well, as it
expects the reply to contain the username exactly as requested.
This patch fixes the problem by making chasquid ignoring the returned
username, which is unused anyway. The protocol is unambiguous enough.
Tests are also amended to always exercise this case.
This patch adds dovecot support to the chasquid daemon, using the
internal dovecot library added in previous patches.
Dovecot support is still considered EXPERIMENTAL and may be reverted, or
changed in backwards-incompatible ways.
The patch also adds the corresponding integration test, which brings up
a dovecot server with a custom configuration, and tests chasquid's
authentication against it. If dovecot is not installed, the test is
skipped.
