implement authentication via K_AUTH

[omar-polo]

This is the continuation of OpenSMTPD/OpenSMTPD-extras#64.

Since proc tables can know do the authentication themselves, it should be possible to handle K_AUTH, receive the plaintext password from smtpd and let the LDAP server do the authentication, instead of the current way of requiring the LDAP server to send us the hashed password.

[likle]

Hey @omar-polo, are you already working on this, or would you like some help with it? I currently see two small challenges in implementing this. First, I believe K_AUTH might not be usable yet, if that gets confirmed. Second, binding the user to verify the password could be problematic due to the single connection and asynchronous nature of table-ldap. One possible solution might be to open a new connection for each request?