... and add "MAILINDEX".

As described in #2789, this is a suggested solution.  Changing UID=xx to
actually get mail with UID xx and add "MAILINDEX" to get a mail with a
special index in the mail box (old behavior).  So MAILINDEX=1 gives the
first non deleted mail in the mail box.

Fixes #2789
Closes #2815

As the email protocols implement SASL authentication rather than IMAP,
POP3 and SMTP specific authentication, updated the authentication
keywords to reflect this.

Added SASL CANCELLATION keywords to differentiate these tests from the
upcoming SASL downgrade tests.

Added support for downgrading the SASL authentication mechanism when the
decoding of CRAM-MD5, DIGEST-MD5 and NTLM messages fails. This enhances
the previously added support for graceful cancellation by allowing the
client to retry a lesser SASL mechanism such as LOGIN or PLAIN, or even
APOP / clear text (in the case of POP3 and IMAP) when supported by the
server.

A failure during authentication, which is performed as part of the
CONNECT phrase (for IMAP, POP3 and SMTP) is considered by the multi-
interface as being closed prematurely (aka a dead connection). As such
these protocols cannot issue the relevant QUIT or LOGOUT command.

Temporarily fixed the test cases until we can fix this properly.