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.