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>

If the input AudioFormat changes but the out_audio_format doesn't
change (e.g. because there is a fixed "format" setting in this
"audio_output" section), the ConvertFilter needs to be reconfigured.
This didn't happen, resulting in awful static noise after changing
songs.

This method is used by DecoderControl::IsCurrentSong(), which is used
by the player thread to check whether the current decoder instance can
be reused to seek.  When switching to another song in the same CUE
sheet, previously DetachedSong::IsSame() returned true, and thus the
old decoder instance was used for the new song, not considering the
new end_time.  This led to the old decoder quickly quitting.

This way, we have four periods instead of the default of two.  With
only two periods, we don't get woken up often enough, and we
frequently encounter buffer overruns.  With four periods, we have more
time to breathe, and the buffer overruns magically disappear.

This setting is mostly useless for capture devices.  There's no point
in configuring it.

This is necessary because we'll never get woken up again by
epoll_wait() after a buffer overrun recovery, unless we start the PCM
explicitly before returning to the I/O loop.

Copy yet more code from the ALSA output plugin.

Copy some good code from the ALSA output plugin.

I want a compiler warning when a new state needs to be considered
here.

Fixes assertion failure in implicit destructor.

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.