Make it possible to feed a string to stdin of a subprocess. Out of
laziness, it can't be an arbitrary byte string. (Would require adding an
option type that takes in a Lua byte string.)
Do not set stdin of a subprocess to fd 0 (i.e. mpv's stdin) anymore,
because it makes things more consistent. Enabling stdin didn't make too
much sense in the first place, so this behavior change seems
justifiable.
win32 support missing.
Fixes: #8003
wm4
e27c523a10