unix: move NETLINK_* consts to own section

This makes it easier to distinguish them from the IFLA_* and add new
entries. Also refresh the IFLA_* consts while at it.

Change-Id: I94aa56c9f99d0714668d8150cb6185d57e2d789c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/622515
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>