xfrm6: Add IPsec protocol multiplexer

This patch adds an IPsec protocol multiplexer for ipv6. 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/ipv6/Makefile
Simple merge

diff --cc net/ipv6/xfrm6_policy.c
Simple merge

diff --cc net/ipv6/xfrm6_protocol.c
Simple merge