include/net/if.h: Add a bit to the device flags to indicate if the device...

include/net/if.h:  Add a bit to the device flags to indicate if the device packet buffer holds an IPv4 or an IPv6 domain packet.  Set/clear the flag along with the correct offset to the application payload data as each packet is received.
net/socket, net/tcp, net/udp:  Add logic to select the domain of the outputgoing packet before sending any UDP or TCP packet.  This sets the bit to indicate the IPv4 or IPv6 domain and the correct offset to the output going payload data.