Fixes regression by commit 4e2a551f

Closes https://github.com/MusicPlayerDaemon/MPD/issues/599

Just in case.

Prepare for a new mode which may convert to a different sample format
when applying volume, to reduce dithering.

It's not a tri-state anymore since we introduced C++ exceptions.

Fixes regression by commit 28e07e90

Closes https://github.com/MusicPlayerDaemon/MPD/issues/596

Pass only the amount of data to PcmExport::Export() when its full
output fits into the ring buffer.  Using only a part of the
PcmExport::Export() result may cause data corruption because
PcmExport's internal state may contain partial blocks which would need
to be rolled back when only some of its output data was used.

As a side effect, this fixes an assertion failure because
PcmExport::CalcInputSize() considered partial block data and could
cause Play() to return a number larger than the "size" parameter.

If snd_pcm_writei() fails, throw an error and stop playback instead
of going into an endless busy loop.

release v0.21.11

Fix src/ls.cxx to only print unique schemas.
Refactor src/ls.cxx to use src/input/InputPlugin functionality.

Add dynamic enumeration support to curl plugin.

MPD used to do that when this code lived in the player thread, but it
was removed by commit 98a7c62d; and
the replacement code in the ALSA output plugin didn't have it.