when probing muxers we don't really need the temporary file to exist in the
filesystem, we can use vlc_memfd() and pass along the file descriptor for
duplication in the file access module.

OutputNew() takes a new 'fd' param, and when probing we use that to pass
in the FD returned by vlc_memfd(), with a NULL for the path. Otherwise we
provide a path and -1 for 'fd'. If 'fd' is -1 it uses 'file' as the access
module and the specified path. If 'fd' is not -1, it uses the FD converted
to a string as the path and 'fd' as the access module. (same module,
different shortcut, which just triggers FD mode via the different access
name).