Fixes build problems with mingw32.

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().

Replaced all occurrences of g_error() with MPD_ERROR() located in a new header
file 'mpd_error.h'. This macro uses g_critical() to print the error message
and then exits gracefully in contrast to g_error() which would internally call
abort() to produce a core dump.

The macro name is distinctive and allows to find all places with dubious error
handling. The long-term goal is to get rid of MPD_ERROR() altogether. To
facilitate the eventual removal of this macro it was added in a new header
file rather than to an existing header file.

This fixes #2995 and #3007.

The replay_gain_state struct holds the precalculated scale factor,
which is removed from struct replay_gain_info.

Don't log "ReplayGain is missing".

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.

The new command "replay_gain_mode" allows the user to switch the
replay gain mode on-the-fly.  No more mpd.conf editing.

There was no setting for disabling replay gain.  It was off when the
"replaygain" setting was not there.

It will be possible to enable replay gain at runtime even when it is
disabled in the configuration file.  This patch enables the preamp
settings in this case.

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.

All config_get_block_*() functions should accept constant config_param
pointers.

Renamed functions, types, variables.

Moved the software volume code to a separate library.

os_compat.h is deprecated and should be replaced with well-defined
standard headers.

Eliminated deprecated libraries.

The currently replay_gain_apply() implementation duplicates code from
pcm_volume(), except that it uses a floating point scale.  Eliminate
all duplicated code from and make it utilize the pcm_volume() library
function.  This introduces replay gain support for 24 bit audio.

Having an array instead of individual variables allows the use of the
replay_gain_mode enum as an array index.

Prefer C enums over CPP macros.

The author's name is already mentioned in the copyright declaration.

Renamed functions and variables.

No CamelCase file names.

Since we use a C99 compiler now, we can assert that the C99 standard
headers are available, no need for complicated compile time checks.
Kill mpd_types.h.