userdb: Add support for receive-only users

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.

internal/userdb/userdb.proto

@@ -12,6 +12,7 @@ message Password {
 	oneof scheme {
 		Scrypt scrypt = 2;
 		Plain plain = 3;
+		Denied denied = 4;
 	}
 }
 
@@ -27,3 +28,5 @@ message Scrypt {
 message Plain {
 	bytes password = 1;
 }
+
+message Denied { }