Mac: Update MoveAndOverwriteFile to use native API

[wilbaker]

Fixes #342.

The initial MoveAndOverwriteFile could result in a missing file (if File.Delete succeeded and File.Move failed).

Switch to the atomic rename function.

DESCRIPTION
     The rename() system call causes the link named old to be renamed as new.  If new exists, it is first removed.  Both old and new must be of the same type (that is, both must be either direc-
     tories or non-directories) and must reside on the same file system.

     The rename() system call guarantees that an instance of new will always exist, even if the system should crash in the middle of the operation.

[sanoursa]

Looks good

[kewillford]

A little weird throwing a Win32Exception from the Mac code but in searching it looks like we are already dependent on that exception.

[sanoursa]

Yep it is weird, but this is what the .NET Core APIs chose to do as well. We just need to translate "Win32Exception" to "NativeException".

[wilbaker]

Yep here is an example:

https://github.com/dotnet/corefx/blob/a10890f4ffe0fadf090c922578ba0e606ebdd16c/src/Common/src/Interop/Unix/System.Native/Interop.ForkAndExecProcess.cs#L44

.NET Core's I/O methods will translate native error codes into a more appropriate exception when possible (e.g. IOException), but in our code we consistently throw Win32Exceptions for all failed native calls.