[chrisuthe]

Adds --release-device / release-device = true. When it is set, the ALSA or PortAudio output is closed when a stream ends and reopened at the next stream/start, so Squeezelite, AirPlay and Spotify can use the same exclusive hw: DAC between Sendspin streams. It is off by default because of the trade-off: another app can take the device between streams, and the next stream is then discarded through the existing per-stream refusal.

• AudioSink::release() defaults to clear(). ALSA and PortAudio override it to drop, close, forget last_format_ and cancel any owed recovery, so poll() never reopens a released device. A late write() discards without logging the device-lost error.
• PlayerListener releases before the stop hook fires, so a hook that starts another player finds the device free.
• On other backends a one-line INFO notice says the option does nothing.
• Tests cover option parsing, config file and bad values, SinkRecovery::cancel(), and the ordering of release vs clear relative to the stop event.

Breaking changes: none for CLI users. The option is opt-in and default behaviour is unchanged.

Manual check still to do (needs hardware): Pi with -o hw:... and --release-device. After a stop, /proc/asound/cardN/pcm0p/sub0/status should read closed, aplay -D hw:... should succeed, and the next Sendspin stream should play.

Closes #80