Temporary hack for the experimental no-GLib build.

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

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

Fixes clang warning.

Fixes clang warning.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Restores the features from the previous draft commands "findin" /
"searchin".

According to the protocol documentation, matching the file name was
wrong.  This removes some awkward special-case code.

Replaces GLib's g_ascii_strcasecmp().

Implement Mantis ticket 0003646.

Merge locate_song_search() and locate_song_match().

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

Convert a string into a tag_type enum.

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.