The recent changes to control message passing broke some things
that get certain types of control messages (ping6 and rtsol are
examples). This gets the new code closer to working:

1. Collect control mbufs for processing in the controlp ==

NULL case, so that they can be freed by externalize.

2. Loop over the list of control mbufs, as the externalize

function may not know how to deal with chains.

3. In the case where there is no externalize function,

remember to add the control mbuf to the controlp list so
that it will be returned.

4. After adding stuff to the controlp list, walk to the

end of the list of stuff that was added, incase we added
a chain.

This code can be further improved, but this is enough to get most
things working again.

Reviewed by: rwatson