Add the SetsockoptCanRawFilter func and CanFilter type to allow kernel
message filtering on AF_CAN sockets.

Fixes golang/go#29638

Change-Id: I4042825527ba2bc9aeb8dbe70d1960a06b15c29e
Reviewed-on: https://go-review.googlesource.com/c/163318
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>