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

Don't use the global variables from the DatabaseSimple library.

Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.

Replaces GLib's GError.

Use the C++ API.

Allow the plugin to allocate the GetSong() return value.

Let the caller take care for legacy conversion.

Preparing for non-"simple" database plugins.

Unused.

Remove GLib dependency from some headers.

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

Add nonnull attributes and fix all callers.

Use this struct for db_walk().

First draft, not really pluggable currently - hard-coded to use the
"simple" plugin, and calls several of its internal functions.

The API is very simple currently, all searches are still performed
over the root "directory" object.  Future changes to the API will move
those search implementations into the plugin, to allow more efficient
implementations, or implementations that don't have the whole tree in
memory all the time.

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.

Do error reporting with GLib's GError library.

Without a music_directory, MPD is an excellent streaming client.

Don't include conf.h in database.c.

Save an empty database, even if the music directory is empty.

Make db_load(), db_save() and db_check() return bool instead of int.

For updating the database, directory_update_init() should be called
explicitly.

"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.

Search'n'replace typo..

Yet another CamelCase removal patch.

The same can be achieved with directory_print(db_get_directory()).

Taming the directory.c monster, part II: move the database management
stuff to database.  directory.c should only contain code which works
on directory objects.