XIA/U4ID: add UDP socket for tunneling

During the loading of a U4ID principal, a socket is created to serve
as the entrance to a tunnel for any encapsulated XIP packets.

By default, this socket is bound to INADDR_ANY:0x41d0 (0.0.0.0:16848),
but can be configured by the network administrator using the kernel
module parameters tunnel_addr (as an IPv4 address string) and
tunnel_port (as a short).
XIA/U4ID: add local U4IDs

Routines for creating, deleting, dumping, and freeing local U4IDs are
added. When a local U4ID is added, a socket is opened on the given
port. The socket is marked as an encapsulation socket, which allows
a callback function, u4id_udp_encap_recv, to be registered, which
will be called upon packet delivery on that socket.

Upon receiving an encapsulated XIP packet, u4id_udp_encap_recv
puts the packet on the loopback device so that the packet is
naturally delivered to the xip_rcv function. This passes control
of the packet from the IP stack to the XIA stack.

A new UDP encapsulation type is added to include/uapi/linux/udp.h.
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.
XIA/U4ID: destroy sockets upon module removal

A linked list of open sockets is maintained so that upon destroying the
network namespace -- or unloading the module -- all of the open U4ID
sockets can be destroyed.