imap does not support PREAUTH

[nmorey]

I'm working on tunneling support for git imap-send (run curl through a preopened socket using
CURLOPT_OPENSOCKETFUNCTION and CURLOPT_SOCKOPTFUNCTION.

The command to open the socket (stdin/stdout) during testing is this one

ssh -q -C localhost /usr/sbin/imapd ./Maildir

It returns:

* PREAUTH [CAPABILITY IMAP4REV1 I18NLEVEL=1 LITERAL+ IDLE UIDPLUS NAMESPACE CHILDREN MAILBOX-REFERRALS BINARY UNSELECT ESEARCH WITHIN SCAN SORT THREAD=REFERENCES THREAD=ORDEREDSUBJECT MULTIAPPEND] Pre-authenticated user [...]

This is not supported by curl although part of the IMAP RFC.

[bagder]

PR #1820 implements a first shot at this, a very basic support of PREAUTH.