GDatagramBased is an interface abstracting datagram-based communications
in the style of the Berkeley sockets API. It may be contrasted to (for
example) GIOStream, which supports only streaming I/O.
GDatagramBased allows socket-like communications to be done through any
object, not just a concrete GSocket (which wraps socket()).
This adds the GDatagramBased interface, and implements it in GSocket.
https://bugzilla.gnome.org/show_bug.cgi?id=697907
