So it turns out that the `purple_proxy_connect_socks5_account` does use the Gio proxy code, but then wraps it up in our old file descriptor concepts. Instead, put that code directly in Jabber, which will eventually make it easier to use straight gio instead of file descriptors.
* Add a cancellable on JabberSIXfer.
* Embed SOCKS5 code directly in jabber SI xfer code.