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.

The volume plugin does not work for reverse_endian samples.

This mixer plugin may be used instead of the traditional global
software mixer.  It integrates with the "volume" filter plugin, and
can control the software volume of an audio output which has no
hardware mixer.

Added public methods to get and set the current volume.

The "volume" filter plugin will replace the current software volume
code.  One "volume" filter may be attached to each output device.
This will allow the user to use hardware mixers for some devices, and
software mixers for other devices at the same time.

Currently, neither the filter API nor the "volume" plugin is
integrated into MPD.

The filter API allows us to implement software volume as a pluggable
filter, and we will be able to integrate libraries like SoX.