Added a new CURLOPT_MAIL_AUTH option that allows the calling program to
set the optional AUTH parameter in the MAIL FROM command.

When this option is specified and an authentication mechanism is used
to communicate with the mail server then the AUTH parameter will be
included in the MAIL FROM command. This is particularly useful when the
calling program is acting as a relay in a trusted environment and
performing server to server communication, as it allows the relaying
server to specify the address of the mailbox that was used to
authenticate and send the original email.