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.

Needed by ax_append_link_flags.m4.

Unused.

Allow using the object outside of DatabaseGlue.cxx.

Remove GLib dependency from some headers.

Produce a smaller binary by removing all unused functions.

Soon we'll use C++11 lambdas which were implemented in gcc 4.5.