and signifincantly improve the readability of ip_input() and
ip_output() again.
The resulting IPSEC hooks in ip_input() and ip_output() may be
used later on for making IPSEC loadable.
This move is mostly mechanical and should preserve current IPSEC
behaviour as-is. Nothing shall prevent improvements in the way
IPSEC interacts with the IPv4 stack.
Discussed with: bz, gnn, rwatson; (earlier version)
