This patch implements method proxy_enumerate from GSocketConnectable for
all connectables (GNetworkAddress, GNetworkService, GInetSocketAddress
and GUnixSocketAddress).
Reviewed-by: Dan Winship <danw@gnome.org>
Higher-level wrappers around GResolver. GSocketConnectable provides an
interface for synchronously or asynchronously iterating multiple
socket addresses, with GNetworkAddress and GNetworkService providing
interfaces based on hostname and SRV record resolution.
Part of #548466.