Use the C++ API.

RTTI adds overhead to the binary, and we don't need that.

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

No more hard-coded "simple" plugin.

Let the caller take care for legacy conversion.

Preparing for non-"simple" database plugins.

Unused.

Allow using the object outside of DatabaseGlue.cxx.

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

New db_get_song() implementation.

Let the caller decide if the current directory should be visited.

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.

For accessing the child of a directory, one needs X_OK on the
directory.

MPD doesn't have child processes anymore, and thus we're not expecting
to receive SIGCHLD very often.  Since hard disk access isn't
interrupted by signals anyway, we don't need those excessive checks.

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.

Check ferror() instead of the fprintf() return value.

Store a list of supported tag items in the database.  When loading a
database which does not have a matching list, we must rescan in order
to get the missing information.

Changed songvec_load() to song_load().  Added start and end markers
for each song.  Removed the "key" line, it's redundant.