We will reuse these tags elsewhere.

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.

The user unit omits the "ProtectKernelModules" setting which fails
with modular kernels:

 Failed at step CAPABILITIES spawning /usr/bin/mpd: Operation not permitted

It is unfortunate that systemd (version 232) is unable to reduce its
own capabilities, because this requires us to split system and user
units.

 https://bugs.musicpd.org/view.php?id=4608

Obsoletes the same variable from AudioConfig.cxx.

Remove dependency on ReplayGain global variables.

Move code from ReplayGainConfig.cxx.

Speed up container_scan() again, by eliminating the need to call
scan_file() for each item.

Prepare for a Decoder API redesign based on an abstract class with
virtual methods.