Now that CloseFilter() may be called again, we can reduce code
duplication.

This allows calling CloseFilter() multiple times.

For easier state management inside filter plugins.

This only applies to linux systems.  Here, sched_setscheduler() is
called to get realtime scheduling.  With this patch, the return value
of this function is now checked and a warning / error message is
generated if it fails.

Works around the "PCM conversion from f to dsd is not implemented"
error message that prevents DSD playback.

Fixes memory leak because ao_plugin_close() never gets called.

Be consistent.

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

Allows the kernel to combine timer wakeups with other processes,
reducing power usage.

For debugging.

Use the existing PcmDither library.

Since opening the PcmVolume object can now fail, this case must be
handled.