CertOpenStore's hCryptProv parameter should get a [Reserved] attribute

[AArnott]

As @drauch points at in microsoft/CsWin32#1002, and backed by our documentation, hCryptProv is meant to be NULL for all users. Can we add the [Reserved] attribute to that parameter?

This will allow CsWin32's friendly overload to omit the parameter.

[riverar]

Warning: This is not valid for Windows Server 2003 and Windows XP. This runs into an open question-- which OSes does win32metadata target? That answer could help define the scope of the supported OS attribute too. #738

[AArnott]

Ah, I hadn't noticed that. Thanks for pointing it out.

In general, if the parameter isn't always reserved, we shouldn't attribute it, and instead leave it to the author to do the right thing.
But in the case of XP and 2003, I personally think we should only support Windows versions that Microsoft supports.

[mikebattista]

I'm fine keeping Reserved in this case.

[riverar]

I don't think we should be entangling metadata with whatever Microsoft's support policy is today. By marking a possibly reserved parameter as [Reserved] we then create an issue where projections can't differentiate between this reserved parameter and one that truly is always null (even on Windows XP).