unix: add IFAL_* consts and ifaddrlblmsg on linux

Implments consts and structs related to ifaddrlabel

Change-Id: I6dd78e74a8e32a26286b95b0a7a20343cb16a446
GitHub-Last-Rev: 11be45c36dcbd7aa31009dfaf966eaebcbf9bbb0
GitHub-Pull-Request: golang/sys#252
Reviewed-on: https://go-review.googlesource.com/c/sys/+/683775
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>