Allows database plugins to use the EventThread, e.g. for CURL
integration.

The old struct config_param remains only for top-level string options.

The new method IsPlugin() replaces the "is_simple" flag.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Allow database plugins to announce that they have been modified.

This variable is initialized once on startup.  It will never change.

Replaces GLib's GError.

Allow using the object outside of DatabaseGlue.cxx.

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.

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.