This fixes the (mostly harmless but nevertheless undefined behaviour)
data race on the output file descriptor.

This is simpler and faster (as we get line buffering for free).

(no functional changes)

Use the dedicated player callback. Don't wait for user input.

Line-based buffering was enabled in 19ea8feb.

(Beware the locale decimal separator.)

Not sure what it meant, probably a short for status. But since status
is printed asynchronously nowadays, it serves no purpuse.

The player lock is already serialising all the code paths.

We already have the player state callback.

Make sure the state of the player when the interface state is printed
before any other state update.