Update TIdSMTPServer to support the [initial-response] parameter of RFC 2554 Section 4

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TIdSMTPServer does not currently support the optional initial-response 
parameter when a client sends an AUTH LOGIN command.  Exchange server 
sometimes uses this parameter when forwarding emails to other servers 
while configured to use AUTH.

According to RFC 2554 Section 4:

AUTH mechanism [initial-response]

The optional initial-response argument to the AUTH command is
used to save a round trip when using authentication mechanisms
that are defined to send no data in the initial challenge.
When the initial-response argument is used with such a
mechanism, the initial empty challenge is not sent to the
client and the server uses the data in the initial-response
argument as if it were sent in response to the empty challenge.
Unlike a zero-length client answer to a 334 reply, a zero-
length initial response is sent as a single equals sign ("=").
If the client uses an initial-response argument to the AUTH
command with a mechanism that sends data in the initial
challenge, the server rejects the AUTH command with a 535
reply.

An example of such a failed login is demonstrated at:

http://www.mail-archive.com/xmail@xmailserver.org/msg15570.html

Original issue reported on code.google.com by gambit47 on 9 Mar 2010 at 7:05

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Original comment by gambit47 on 13 Mar 2010 at 10:12

• Changed state: Started

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Implemented in rev 4459.

Original comment by gambit47 on 12 Dec 2012 at 8:04

• Changed state: Verified