Eliminates Complete().

Commit 3a212412 changed the meaning of out_audio_format, but I forgot
to apply this to ConvertFilter::FilterPCM().

See https://bugs.musicpd.org/view.php?id=4551

Previously, volume was applied twice: once by PcmVolume, and again by
the hardware mixer.

For easier state management inside filter plugins.

The old struct config_param remains only for top-level string options.

Fixes -Winconsistent-missing-override (clang 3.6).

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

Merge duplicate code.