This change adds the Marshal method of RouteMessage to make it
possible to exchange route messages between userspace processes and
the kernel for the manipulation of routing information base inside the
kernel.

Change-Id: I0cf2c1a391820f41eb9c5eac1c172598cb2e1533
Reviewed-on: https://go-review.googlesource.com/36077Reviewed-by: Ian Lance Taylor <iant@golang.org>