Investigate ACL APIs

[weshaggard]

The AccessControl APIs are windows specific but cross-cutting across various other APIs in the standard. We need to take a closer look at what APIs get broken if we remove them from the standard.

[weshaggard]

I put together PR #21 to show what breaks would occur if we removed the ACL APIs from netstandard.

We have the following options:

1. Keep the minimal closure in netstandard so we don't have to remove any members. I suspect the closure will be close to all the ACL types. This will also cause them to be at least stubbed out to throw on other platforms.
2. Live with the removed members and people wanting to use them will either need to build against the specific platform that has them (i.e. cross-compile) or use some new extensions like corefx has created. Using the extensions will get most of the scenarios supported but we won't be able to handle constructors or static APIs in this manor.

I'm on the fence with this one because on the one hand the ACL API's aren't supported on non-windows but it is pretty breaking across many APIs for windows developers if we remove them.

[weshaggard]

After discussion with FXDC we decided to go with option (2) and remove the ACL API's from the standard. The general consensus is that they are low usages and only work on Windows so we believe it should be OK to factor these out into an extension on top of netstandard. The extension will not contain 100% of the APIs but it will contain enough to enable all the scenarios for anyone that needs to use them when targeting netstandard and running on windows. Folks can also target .NET Framework directly if they need all these APIs.

[weshaggard]

PR #21 removes the ACL APIs from netstandard.