* smtp.el: Add autoload settings for `starttls-open-stream' and

* smtp.el: Add autoload settings for `starttls-open-stream' and
`starttls-negotiate'.
(smtp-connection-set-extensions-internal): New macro.
(smtp-connection-extensions-internal): New macro.
(smtp-make-connection): Set the `extension' slot to nil.
(smtp-primitive-ehlo): New function.
(smtp-submit-package): Rename from `smtp-commit'.
(smtp-submit-package-function): Rename from `smtp-commit-function'.
(smtp-primitive-starttls): New function.
(smtp-extensions): New group.
(smtp-use-8bitmime): New variable.
(smtp-use-size): New variable.
(smtp-use-starttls): New variable.
(smtp-via-smtp): Bind `smtp-open-connection-function'.