In the async case, a failed DNS lookup was causing the proxy
resolution to bail out immediately, rather than just moving on to the
next potential proxy (which might not need us to do the DNS lookup
beforehand). Fix that.
An implementation of GSocketAddressEnumerator that handles proxy
enumeration. This class is mainly usefull for Connectables implementation
such as NetworkService, NetworkAddress and SocketAddress to handle proxies.
Reviewed-by: Dan Winship <danw@gnome.org>
