xfrm4: Add IPsec protocol multiplexer

This patch add an IPsec protocol multiplexer. With this
it is possible to add alternative protocol handlers as
needed for IPsec virtual tunnel interfaces.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

diff --cc include/net/xfrm.h
Simple merge

diff --cc net/ipv4/Makefile
Simple merge

diff --cc net/ipv4/xfrm4_input.c
Simple merge

diff --cc net/ipv4/xfrm4_protocol.c
Simple merge

diff --cc net/xfrm/xfrm_input.c
Simple merge