XIA/U4ID: add local U4IDs

Routine 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: added main U4IDs, fixed local U4IDs