This patch also updates corresponding functions from
`LibFileSystemAccessServerClient`.
From the FileSystemAccessClient point of view, it only makes the server
take `Core::Stream::OpenMode` instead of `Core::OpenMode`. So, `enum`
conversions only happen within deprecated functions and not in the new
`Core::Stream` friendly API.
On the server side, it just removes two usages of `Core::File::open()`.
c0bc3b9814