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

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.

Overload the name.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Replace g_strchug() calls with a cheaper implementation.

Use the functions song_save() and song_load() to use the same format
as in the database file for those songs which need the tags.

While this is not useful for the database, it may become useful for
reusing this code for the state file.

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.

Convert a string into a tag_type enum.

Clear the colon.  This simplifies all attribute parsers, because they
can now use strcmp() instead of strncmp().

Fix a minor memory leak.

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