* smtp.el: Require `net-trans'.

* smtp.el: Require `net-trans'.
(smtp-transaction): New class.
(smtp-open-connection-function): New variable.
(smtp-default-commands): New variable.
(smtp-commands): New variable.
(smtp-transaction-function): New variable.
(smtp-greeting): New generic function.
(smtp-ehlo): New generic function.
(smtp-helo): New generic function.
(smtp-mailfrom): New generic function.
(smtp-rcptto): New generic function.
(smtp-data): New generic function.
(smtp-via-smtp): Simplify.
(smtp-check-response): New function.