Fixed incorrect error code being returned in STARTTLS

The STARTTLS response code in SMTP, POP3 and IMAP would return
CURLE_LOGIN_DENIED rather than CURLE_USE_SSL_FAILED when SSL/TLS
was not available on the server.

Reported by: Gokhan Sengun
Bug: http://curl.haxx.se/mail/lib-2012-01/0018.html