Disallow insecure protocols during autoconfiguration by default.

Introduce an option mailnews.auto_config.ssl_only (default:
true) which, when set, affects autoconfiguration as follows:

* Only allow Mozilla database lookups if the URL uses HTTPS.
* Only try ISP lookup over HTTPS.
* Discard configurations not using SSL/TLS or STARTTLS from the
  above two methods.
* Only probe SMTP/POP/IMAP with SSL/TLS and STARTTLS.

In other words, when the option is set, enforce secure protocols
both when doing lookups and in the resulting configuration. Without
this, there are several vectors for MitM during autoconfiguration.
In the autoconfiguration window the option can be toggled via a
checkbox so that legacy ISPs that only support plaintext protocols
are still supported. (Will-fix: #971347)

In order to better support privacy oriented tools like Tor, add
SOCKS support during the autoconfiguration probing step, which
previously was always performed unproxied even when a proxy was
configured. (Will-fix: #669238)

Also, introduce an option mailnews.auto_config.dns_mx_lookup.enabled
(default: true) which controls whether the DNS MX lookup step should
be performed during autoconfiguration.