Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

The problem with std::queue is that it doesn't give us enough control.
The method Clear() is a kludge already, but soon, we'll need
filtering.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

Add UpdateQueueItem::id to keep track of the id in every item.
Replaces thhe hack in update_queue_push().

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.

"rescan" is the same as "update", but it discards existing songs in
the database.

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.

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.

Added a small RIFF parser library.  Look for an "id3" chunk, and let
libid3tag parse it.

Moved everything related to saving or loading the playlist from/to the
state file to playlist_state.c.

In generic PCM functions allowing all sample formats, pass a void
pointer instead of a char pointer.

Moved the software mixing code (used by crossfading) to a separate
library.

Don't use libc's rand() function, because it is slow.  Our own trivial
linear congruential generator is good enough for dithering.

redirect_stdin() is a daemonization function, and disconnecting from
the standard input is always a good idea for MPD.

Prepare for the migration to the GLib option parser, which uses
gboolean for flags.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

When there are standardized headers, use these instead of the bloated
os_compat.h.