Rewrite of the pc_get_error_message() function, now using a GError
object instead of the complicated "errored_song" attribute.

Eliminates explicit global initialisation.

The new block overrides the "db_file" setting, and allows configuring
any database plugin.

No more hard-coded "simple" plugin.

Let the caller take care for legacy conversion.

Preparing for non-"simple" database plugins.

Useless call.

Fixes assertion failures in the Directory library.

Implements error checking for the gmtime_r() call, which was missing
in two code locations.

Implementation incomplete, but sort-of-works.  DumpDatabase works, but
MPD is still hard-coded on the "simple" plugin.

Fixes regression.

Let the database plugin do the match.

Merge locate_song_search() and locate_song_match().

Fold the case during construction, without having to create another
copy.

Prepare to use the DatabasePlugin C++ API directly.

Prepare for splitting command.c.