This functionnallity can be disabled using property enable-proxy. It
enumerates addresses using GSocketConnectable::proxy_enumerate() instead of
enumerate(). When the returned address is of type GProxyAddress (a type
based on GInetSocketAddress), it gets the proxy protocol handler using
g_proxy_get_default_for_protocol() and call connect() on it.

Reviewed-by: Dan Winship <danw@gnome.org>