That seem wrong.  The proper solution is to add a PR_PURGEIF flags and
  add that to the appropriate protosw entries.

That makes sense, and is probably easier.  It also has the advantage
of failing in the same way as the system currently does for odd
protocols; I might miss changing protocols I don't have compiled in (I
just have INET, INET6, and a BBN-developed subnetwork-layer protocol
in my kernels).

-- 
        Greg Troxel <gdt@ir.bbn.com>