Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

We will reuse these tags elsewhere.

The first step to multi-player support.  Not much, just a dummy
command.

Eliminate global variables.

Use SND_PCM_NONBLOCK, and perform all snd_pcm_writei() calls in the
IOThread.  Use a lockless queue to copy data from the OutputThread to
the IOThread.

This rather major change aims to improve MPD's internal latency.  All
waits are now under MPD's control, instead of blocking inside
libasound2.

As a side effect, an output's filter is now decoupled from the actual
device I/O, which solves a major latency problem with the conversion
filter on slow CPUs and small period buffers.  See:

 https://bugs.musicpd.org/view.php?id=3900

handle_tagtypes() is also being moved to ClientCommands.cxx, because
that command will be extended to access client-specific settings.

The internal static xiph library needs to link with libogg. Otherwise
building mpd will fail:

```
/mips-linux-gnu/bin/ld: libxiph.a(libxiph_a-OggVisitor.o): undefined
reference to symbol 'ogg_stream_packetout'
```
Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>

Move code from SongFilter.cxx.

source:
http://git.alsa-project.org/?p=alsa-utils.git;a=blob_plain;f=alsamixer/volume_mapping.c;hb=HEAD
http://git.alsa-project.org/?p=alsa-utils.git;a=blob_plain;f=alsamixer/volume_mapping.h;hb=HEAD

Eliminate one unnecessary dependency for debug programs which don't
need the InputStream API.

Problems found by "make distcheck".

The CurlRequest gives us a more low-level API to CURL without the
InputStream interface, integrated into our IOThread.