The Song class is only used for database songs now.  A Song without a
Directory is not possible anymore.

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.

Not necessary anymore for playlist_metadata_load().

Replaces GLib's g_str_has_prefix().

Replaces GLib's GError.

Wrap songvec_find() and other songvec methods.

Random access is not needed, and a linked list is easier to manage: we
don't need to (re-)allocate the pointer array.

Pass only the "name" to a directory, instead of the full (relative)
path.

The function playlist_metadata_load() will overwrite the input buffer
before using the "name" parameter; since "name" points to the same
buffer, we'll get a corrupted string.

Don't open the music directory for each "lsinfo" call.  Get the list
of playlist files from the memory database.

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.

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

Use a single GString buffer object in all functions loading the
database.  Enlarge it automatically for long lines.  This eliminates
the maximum line length for tag values.  There is still an upper limit
of 512 kB to prevent denial of service, but that's reasonable I guess.

Allocate the directory object after the "directory:" line.  Assign the
mtime from the input file to this new object, instead of to the parent
directory.

Fix a minor memory leak in the error handler.

The old code tried to recover, but what's the point of that?  If a
directory is duplicate, something is wrong with the database file.