introduce ipip tunnel support

This patch introduces ipip tunnel support. This
works with kernel module ipip.

Signed-off-by: Susant Sahani <susant@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
[thaller@redhat.com: change WS to TAB, fix code doc]
Signed-off-by: Thomas Haller <thaller@redhat.com>