pop3: Added support for enabling the SASL initial response

Allowed the user to specify whether to send the client's intial response
in the AUTH command via CURLOPT_SASL_IR.
smtp: Added support for disabling the SASL initial response

Updated the default behaviour of sending the client's initial response in the AUTH
command to not send it and added support for CURLOPT_SASL_IR to allow the user to
specify including the response.

Related Bug: http://curl.haxx.se/mail/lib-2012-03/0114.html
Reported-by: Gokhan Sengun
imap: Added support for overriding the SASL initial response

In addition to checking for the SASL-IR capability the user can override
the sending of the client's initial response in the AUTHENTICATION
command with the use of CURLOPT_SASL_IR should the server erroneously
not report SASL-IR when it does support it.
tests: Updated SMTP tests to decouple client initial response

Updated test903 and test904 following the addition of CURLOPT_SASL_IR
as the default behaviour of SMTP AUTH responses is now to not include
the initial response. New tests with --sasl-ir support to follow.