command: add a way to abort asynchronous commands

Many asynchronous commands are potentially long running operations, such
as loading something from network or running a foreign process.
Obviously it shouldn't just be possible for them to freeze the player if
they don't terminate as expected. Also, there will be situations where
you want to explicitly stop some of those operations explicitly. So add
an infrastructure for this.

Commands have to support this explicitly. The next commit uses this to
actually add support to a command.

DOCS/client-api-changes.rst

@@ -36,6 +36,7 @@ API changes
  1.102  - redo handling of async commands
         - add mpv_event_command and make it possible to return values from
           commands issued with mpv_command_async() or mpv_command_node_async()
+        - add mpv_abort_async_command()
  1.101  - add MPV_RENDER_PARAM_ADVANCED_CONTROL and related API
         - add MPV_RENDER_PARAM_NEXT_FRAME_INFO and related symbols
         - add MPV_RENDER_PARAM_BLOCK_FOR_TARGET_TIME