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.

Really drain all data from the period_buffer in any case.

This assertion is not about whether all data has been consumed, but
about whether there has been any data at all.

Draining isn't finished just because the period_buffer has run empty.
It is only finished after snd_pcm_drain() has succeeded.

Apparently, if snd_pcm_drain() returns EAGAIN, it does not actually
want to be called again; the next call will snd_pcm_drain() will also
return EAGAIN, forever, even though the PCM state has meanwhile
switched to SND_PCM_STATE_SETUP.  This causes a busy loop; to fix
this, we should always check snd_pcm_state() to see if draining is
really required.

There cannot be partial frames in the buffer, so we don't need
GetPeriodPosition(); it's enough to check whether head has been moved
in this period.

This gives MPD more control, because attempts to avoid having partial
periods in the ALSA period buffer.  For example, this means that
DrainInternal() doesn't need to generate silence to fill the partial
period.

See https://github.com/MusicPlayerDaemon/MPD/issues/545

Fix build breakage from commit ea639269

Closes https://github.com/MusicPlayerDaemon/MPD/issues/589

This makes the configuration error more visible, possibly addressing
one part of https://github.com/MusicPlayerDaemon/MPD/issues/589

Commit 1eae9339 added support for
multiple "groups" in the "list" command, and this change allows
clients to detect that this behavior, which had been documented for
several years, is now implemented properly.

Combining these two in one single value saves some memory, but is
complicated and we may need the src_sample_format for new features
later.

This reverts commit 2c3eeb71.  This
workaround has been obsoleted by commits
bf26adf5 and
32380d1d

Similar to commit 32380d1d, these are
the final parts for really fixing
https://github.com/MusicPlayerDaemon/MPD/issues/469