Eliminate SMTP AUTOMATIC authentication

The server settings for IMAP and POP3 have no such AUTOMATIC setting.
(Nor does Thunderbird have any such setting.)

The AUTOMATIC option is no longer offered to users as a choice.  A
pre-existing setting will continue to be honored, but only to the extent
that it doesn't result in insecure password transmission.  Users in such a
situation will get a "Failed to send some messages" notification
containing the exception text that says to update their outgoing server
authentication setting.

One of the problems with "AUTOMATIC" is that users may not fully
understand its security implications.  For example, a MITM attack could
mask a server's support for STARTTLS and CRAM-MD5, resulting in password
disclosure in certain configurations.

This commit also makes changes to the SMTP authentication process.  No
attempt is made to authenticate using methods that the server does not
profess to support in its EHLO response.  This is the same behavior as
found in Thunderbird.