The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

Don't depend on the "simple" database plugin.  This fixes an assertion
failure / crash and allows using stickers with other plugins.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Replaces GLib's GError.

Reduce the number of lock/unlock cycles, and make database handling
safer.

sticker_delete_value() deletes only one value in a sticker, while
the old function sticker_delete() deletes all values.

This is a wrapper for sticker_find(), which looks up the song object.

sticker_list_values() is only used internally in sticker.c.  Remove
sticker_song_list_values() completely, it is superseded by
sticker_song_get().

The sticker struct can be used for enumerating values.  This will
replace the sticker_list_values() function.

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.

[mk: merged memory leak patch; fixed indentation (tabs); fixed
documentation typo]

This is a wrapper for the backend sticker library.  It provides
several glue functions which take a song object instead of type/uri.

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.