command: extend subprocess command stdin, change behavior

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

DOCS/interface-changes.rst

@@ -74,6 +74,9 @@ Interface changes
     - undeprecate --video-sync=display-adrop
     - deprecate legacy auto profiles (profiles starting with "extension." and
       "protocol."). Use conditional auto profiles instead.
+    - the "subprocess" command does not connect spawned processes' stdin to
+      mpv's stdin anymore. Instead, stdin is connected to /dev/null by default.
+      To get the old behavior, set the "passthrough_stdin" argument to true.
  --- mpv 0.32.0 ---
     - change behavior when using legacy option syntax with options that start
       with two dashes (``--`` instead of a ``-``). Now, using the recommended