Expose some of the settings used in the disable_standby_mode.

Specifically, firstly, the disable_standby_mode_silence_threshold, which is the
amount of audio in the output device's hardware buffer. It should normally
be close to the value givein in the audio_backend_buffer_desired_length_in_seconds
setting. If it drops to this value, silence is added to the buffer to prevent
the output device from becoming idle.

Secondly, the disable_standby_mode_silence_scan_interval is the time between checks
of the output device's hardware buffer.

The code for generating frames of possibly-dithered silence has been simplified.

scripts/shairport-sync.conf

@@ -93,6 +93,8 @@ alsa =
 //	maximum_stall_time = 0.200; // Use this optional advanced setting to control how long to wait for data to be consumed by the output device before considering it an error. It should never approach 200 ms.
 //	use_precision_timing = "auto"; // Use this optional advanced setting to control how Shairport Sync gathers timing information. When set to "auto", if the output device is a real hardware device, precision timing will be used. Choose "no" for more compatible standard timing, choose "yes" to force the use of precision timing, which may cause problems.
 //	disable_standby_mode = "never"; // This setting prevents the DAC from entering the standby mode. Some DACs make small "popping" noises when they go in and out of standby mode. Settings can be: "always", "auto" or "never". Default is "never", but only for backwards compatibility. The "auto" setting prevents entry to standby mode while Shairport Sync is in the "active" mode. You can use "yes" instead of "always" and "no" instead of "never".
+//	disable_standby_mode_silence_threshold = 0.040; // Use this optional advanced setting to control how little audio should remain in the output buffer before the disable_standby code should start sending silence to the output device.
+//	disable_standby_mode_silence_scan_interval = 0.004; // Use this optional advanced setting to control how often the amount of audio remaining in the output buffer should be checked.
 };
 
 // Parameters for the "sndio" audio back end. All are optional.