to protocol handlers, based on src/dst (for ip proto #4/41).
see comment in ip_encap.c for details of the problem we have.
there are too many protocol specs for ip proto #4/41.
backward compatibility with MROUTING case is now provided in ip_encap.c.
fix ipip to work with gif (using ip_encap.c). sorry for breakage.
gif now uses ip_encap.c.
introduce stf pseudo interface (implements 6to4, another IPv6-over-IPv4 code
with ip proto #41).
