Some use cases, like receive-only MTAs, need domain users for receiving
emails, but have no real need for passwords since they will never use
submission.
Today, that is not supported, and those use-cases require the
administrator to come up with a password unnecessarily, adding
complexity and possibly risk.
This patch implements "receive-only users", which don't have a valid
password, thus exist for the purposes of delivering mail, but always
fail authentication.
See https://github.com/albertito/chasquid/issues/44 for more details and
rationale.
Thanks to xavierg who suggested this feature on IRC.
