unix: add pidfd constants and type

Change-Id: Ide4fd8c973c214b28f0ebfec3cd442052323f934
Reviewed-on: https://go-review.googlesource.com/c/sys/+/817380
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>