imap/smtp: Fixed incorrect SASL mechanism selection with XOAUTH2 servers

XOAUTH2 would be selected in preference to LOGIN and PLAIN if the IMAP
or SMTP server advertised support for it even though a user's password
was supplied but bearer token wasn't.

Modified the selection logic so that XOAUTH2 will only be selected if
the server supports it and A) The curl user/libcurl programmer has
specifically asked for XOAUTH via the ;AUTH=XOAUTH login option or 2)
The bearer token is specified. Obviously if XOAUTH is asked for via
the login option but no token is specified the user will receive a
authentication failure which makes more sense than no known
authentication mechanisms supported!