dgram: change Socket.bind() to return itself

This commit changes `lib/dgram.js` Sockets to, when
they are bound to a port / IP, return themselves. This
is done in order to allow chaining of methods and be
in accordance with the `lib/net.js` library.
dgram: changes Socket.close() to return itself

This commit adds a return statement to the dgram.Socket.close()
function that returns itself after it finishes. This follows along
the functionality of the more popular and, dare I say, father-library
`lib/net.js`.