Previously, you could only provide a CRT FD, which works great for the
most part, but when using CreateProcess on Windows, it requires the use
of the reserved fields of STARTUPINFO to inherit the FD to a child mpv
process. This becomes a problem for example if you use Go's os/exec API,
which does not expose a way to set those reserved fields. Exposing a way
to allow passing handles directly fixes the mentioned scenario, and
makes --input-ipc-client more flexible at no cost.
