windows: add functions for dealing with elevated tokens
windows: add functions for dealing with elevated tokens

These are required when dealing with UAC or launching processes as
elevated administrators on behalf of other users.

Change-Id: If256c838b1f0202a8703d91496ffdbe16be3a700
Reviewed-on: https://go-review.googlesource.com/c/sys/+/176858
Run-TryBot: Jason Donenfeld <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>

golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys