There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Replaces struct playlist_provider.

The "bool" type is built-in.

Reduce header dependencies.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.

Optionally allow all local files.

"Insecure" mode is used for printing playlists.

Prepend the playlist's base URI to relative song URIs.  Look up songs
in the database (if the URI refers to a local song file).  Merge
existing database metadata with metadata from the playlist plugin.

This patch integrates the playlist plugin API to the MPD core.  We'll
be able to do much more in the future with that API, that's just the
beginning.

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.

The output_command library provides a command interface to the audio
outputs.  It assumes the input comes from an untrusted source
(i.e. the client) and verifies all parameters.

Added audio_format_parse() in a separate library, with a modern
interface: return a GError instead of logging errors.  This allows the
caller to deal with the error.

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.

Similar to decoder_control.c, output_control.c will provide functions
for controlling the output thread (which will be implemented later).

This function is declared, but is neither used nor implemented.

Instead of having to register each output plugin, store them
statically in an array.  This eliminates the need for the List library
here, and saves some small allocations during startup.

Also rename AudioOutputPlugin to struct audio_output_plugin, and use
forward declarations to reduce include dependencies.

Just like decoder_api.h, output_api.h provides the audio output API
which is used by the plugins.