windows: support SECURITY_DESCRIPTOR and ACL for secured objects

This adds the basic foundation for dealing with security descriptors and
access control lists. The basic creators and getters are included in
this patch. These are some of the most fundamental security objects on NT,
and any work with the security API is fairly limited without it. These
are "core" NT structures.

Change-Id: I9a6399cb6ee41a825de30d5364ab69102d5f6d57
Reviewed-on: https://go-review.googlesource.com/c/sys/+/195498
Run-TryBot: Jason A. Donenfeld <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Jason A. Donenfeld <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>