DGD Network extensions

The network extensions allow LPC code to open ports for
listening, to make outbound tcp connections, and to 
send and receive udp without the restrictions dgd normally
puts on this.

The network extensions are disabled by default, and must be
turned on at compile time by adding -DNETWORK_EXTENSIONS
to the DEFINES in the Makefile so:

  DEFINES=-D$(HOST) #-DNETWORK_EXTENSIONS #-DDUMP_FUNCS

becomes

  DEFINES=-D$(HOST) -DNETWORK_EXTENSIONS #-DDUMP_FUNCS

Turning on the network extensions may turn your mud into
a viable platform for attacking other systems, so care
should be taken to properly secure access to the related
kfuns.

When enabling the network extensions, DGD's mechanism for
listening for inbound connections changes, your lib must
be ready for this.

See svn://wotf.org/kernellib-net/ for an example.

Also, the network extensions disable ipv6 support.

The network extensions add and change a number of kfuns:

  connect
  open_port
  ports
  send_message
  send_datagram

And disable the datagram_challange kfun.