Switch IfMsghdr and IfaMsghdr to their 'l' variants, make the IfData layout
to be based on FreeBSD-11.0 (freebsdVersion >= 1100011).
Using freebsdVersion, detect the appropriate layout at runtime and decode
routing socket messages into the new IfData layout.

Fixes #11641

Change-Id: Ic7ec550f00c0d15f46a36f560d835e4f138f61e1
Reviewed-on: https://go-review.googlesource.com/14757Reviewed-by: Russ Cox <rsc@golang.org>