Fixes the dreaded error "Failed to create : No such file or
directory".

 https://bugs.musicpd.org/view.php?id=4625

The unit test was wrong as well.  D'oh!

The byte order of DSD_U32 was wrong from the start.  The oldest bits
must be in the MSB, not in the LSB, according to
snd_pcm_format_descriptions in alsa-lib.

These were disabled by accident.

We'll have some code for it soon.

Hook for src_reset(), not yet used.

Move this sample rate fixup from the ALSA output plugin to PcmExport,
where it belongs.

DSD_U32 packs four bytes instead of one large "sample", thus the
sample rate is one quarter of the input sample rate.  This fixes a
rather critical DSD_U32 playback problem.

Prepare for DSD sample rate fixups.

https://bugs.musicpd.org/view.php?id=3876

Call SeekDone() to avoid the freeze bug.

libcurl's WRITEFUNCTION is pretty fragile; if we destroy the CURL*
instance or even unregister it using curl_multi_remove_handle(),
libcurl will crash instantly.  But still we need to be able to handle
exceptions from inside the WRITEFUNCTION, and call
CurlResponseHandler::OnError(), which may destroy the whole thing.  As
a workaround, I use DeferredMonitor to postpone the OnError() call
into a stack frame which is allowed to destroy the request.