This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The libWavPack documentation says:

 "After a FALSE return the file should not be accessed again (other
 than to close it); this is a fatal error."

Technically, these samples are signed.  There was no practical
difference, only the declaration was formally wrong.

Since we don't use OPEN_2CH_MAX, this should be equal to
WavpackGetNumChannels() - but who knows, maybe a future libWavPack
version breaks this assumption.

Enable OPEN_DSD_AS_PCM if available.  No OPEN_DSD_NATIVE yet.

 https://bugs.musicpd.org/view.php?id=4606

Use MPD's internal APE tag code, which works just as well.  This
removes some duplicate code for reading tags and ReplayGain.

MPD uses a floating point range from -1 to +1 internally - why ask
libwavpack to use -32768..+32768 only to reduce it back to -1..+1
afterwards?

Declare two named elements.  An enum should not be used as an array
index, as this is error prone.

Replaces decoder_data() and others.

Replaces decoder_initialized().

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.

Let std::unique_ptr manage both the InputStream and the WavpackInput.