The standard has been renamed since the early draft that was
implemented in MPD.

Translate SampleFormat::DSD to SND_PCM_FORMAT_DSD_U8, which was added
to alsa-lib 1.0.27.1.

Make it "const".

See code comment.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Workaround for driver bug observed on the Raspberry Pi, see code
comment for details.

Fixes nullptr dereference in std::string::operator=().  Also fixes a
memory leak by using config_get_block_string() instead of
config_dup_block_string().

.. and not in_frame_size, because this relates to the frame size being
sent to ALSA.  pcm_export_source_size() will then turn it back into
the in_frame_size scale.

Move code from the ALSA output plugin.

The hard-coded "3 bytes" was wrong because it ignored the number of
channels.

DSD-over-USB should not be a MPD core format, because it is not a
"natural" format; it is just a temnporary over-the-wire format.  This
format has been implemented in pcm_export, and does not need to be
supported by pcm_convert.