Default to TLS for https connections, and fall back to SSLv3 on failure

Default to TLS for https connections, and fall back to SSLv3 on failure

Rather than always using SSLv3, try proper TLS+extensions first, and
fall back to SSLv3-without-extensions if that gives an error that
looks like it might mean "broken SSLv3-only server". Use
SoupSessionHost to record the fallback status for a host.

https://bugzilla.gnome.org/show_bug.cgi?id=581342