XIA/U4ID: add u4id_deliver and DST I/O functions

If the U4ID to be routed is a local U4ID and it is not a sink, we
have reached the destination of an XIP tunnel. Therefore, the last
node should be advanced and routing may continue.

If the U4ID to be routed is a main U4ID and it is not a sink, we
need to tunnel:

If the packet needs to be written to the network, then an output
function is called. The output functions tunnel the XIP packet by
fetching the IPv4 address and UDP port from the XID and adding
an IP and UDP header to the packet before transmitting it.

If the packet is being received, then an input function is
called. The input functions expand the struct sk_buffer if
necessary, and if the packet came from the network then the
TTL is decreased. The output function is then invoked to write
the packet to the network.
XIA/U4ID: remove requirement to add main table entries

By removing the ability to add main table entries, network administrators
do not have to explicity add the destination of the ephemeral tunnel to
the forwarding table.