By using native rmdir(2) syscalls when deleting placeholder
directories, we can specifically detect the non-empty directory
error condition ENOTEMPTY, which is otherwise folded into a
generic IOException by Directory.Delete().

We need to return EDirectoryNotEmpty in this case because the
calling code in GitIndexProjection handles this result as an
expected and non-exceptional condition.

To simplify our code we then also use unlink(2) to delete files
in a common NativeMethods subclass.

We further detect the case where an inode is neither a file
nor a directory (libprojfs's API will return EPERM for such inodes)
by introducing an Unknown projection state and reporting this as
a "full file" condition to our caller, in the same manner as the
macOS PrjFSLib C++ implementation.

Finally, we handle requests to delete the repository's root
directory by returning EDirectoryNotEmpty, instead of ignoring
them and returning Success.  This is necessary to avoid short-
circuiting the logic of the calling UpdatePlaceholders() method
in GitIndexProjection, which otherwise would fail to correctly
populate a new view of the repository if we return Success when
asked to delete the repository's root.  Since the repository's
root directory is expected to always contain, at a minimum, the
.git subdirectory, EDirectoryNotEmpty is appropriate.

We can return to using managed methods to delete files and
directories when updating projection views on Linux, so long as
we check the IOException's HResult property to detect the
EDirectoryNotEmpty (ENOTEMPTY) condition.

Also set the (unused?) failure cause of ReadOnly when an
access exception occurs.

Thanks to @kivikakk for the hint on checking HResult!

Co-authored-by: Ashe Connor <ashe@kivikakk.ee>