Implement better authentication

Implement better authentication

DMA tries to authenticate by simply trying various authentication
mechanisms. This is obviously not conforming to RFC and some mail
providers detect this is spam and reject all emails.

This patch parses the EHLO response and reads various keywords
from it that can then later in the program be used to jump into
certain code paths.

Currently this is used to only authenticate with CRAM-MD5 and/or
LOGIN if the server supports one or both of these. The
implementation can be easily be extended though.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>