[DGD] connect_datagram()

[Felix A. Croes]

I've added the kfun connect_datagram().  To use it, you need at least
one open datagram port accepting connections.

    connect_datagram(0 /* datagram port index */, remote_host, remote_port);

This establishes an outbound datagram connection without handshake (no
datagrams are actually sent, and in fact it may be impossible to do so).

This completes the replacement of the networking package.  It is also
likely to be the last new kfun for a while.

Regards,
Felix Croes