unix: add SetMemPolicy and its mode/flag values

This reuses CPUSet type as it fits perfectly.

No GetMemPolicy support yet.

Change-Id: I549361bc1ed95ee73fd38d00277ec0f9f348a9ba
Reviewed-on: https://go-review.googlesource.com/c/sys/+/706917
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Antti Kervinen <antti.kervinen@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>