Previously, there was no special code to convert stereo to
multi-channel.  The generic solution for this was to convert to mono,
and then copy the result to all channels.  That's a pretty bad
solution, but at least something which always renders audio.  MPD does
something, instead of failing.

Now that MPD has proper support for multi-channel (by defining the
channel order), we can do better than that.  It is a (somewhat) common
case to play back stereo music on a DAC which can only do
multi-channel.  The best approach here is to copy the stereo channels
to front-left and front-right, and apply the "silence" pattern to all
other channels.

Allow 32 bit platforms to use 32 bit instead of 64 bit for summing 24
bit samples.

This is really just a mono mapper, but the important part is that this
library cannot fail anymore.

Copy the according file from another project (i.e. XCSoar).  This will
allow copying more code more easily.

Allow more compiler optimizations.

Reduces number of implicit integer conversions.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

Don't abort the whole MPD process when the conversion fails.  This has
been a denial-of-service attack vector for years.

You cannot have a negative number of channels, let's pass it as
uint8_t instead of int8_t.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

Some 24 bit code can be reused.  The 32 bit variant has to use 64 bit
integers, because 32 bit integers could overflow.  This may be a
performance hit on 32 bit CPUs.

Replace a "static" buffer with the PCM buffer library.

The 24 bit implementation is mostly copy'n'paste of the 16 bit
version, except that the data type is int32_t instead of int16_t.

Separate code from pcm_utils.c to keep it small and simple.