Don't abuse struct protosw for ip_encap -- introduce struct encapsw.

Mostly mechanical change to replace it, culling some now-needless
boilerplate around all the users.

This does not substantively change the ip_encap API or eliminate
abuse of sketchy pointer casts -- that will come later, and will be
easier now that it is not tangled up with struct protosw.

(riastradh)