Add string serialisation functions for GNetworkAddress, GSocketAddress,
GUnixSocketAddress, GInetSocketAddress, GNetworkService and
GSocketConnectable. These are intended for use in debug output, not for
serialisation in network or disc protocols.

They are implemented as a new virtual method on GSocketConnectable:
g_socket_connectable_to_string().

GInetSocketAddress and GUnixSocketAddress now implement
GSocketConnectable directly to implement to_string(). Previously they
implemented it via their abstract parent class, GSocketAddress.

https://bugzilla.gnome.org/show_bug.cgi?id=737116