Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

If a directory is a mount point, omit the "directory: " as well.

This bug is years old, but has become more visible now that mount
points are persistent in the state file.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Remove the C list_head library and use type-safe C++ instead.

A SimpleDatabase instance can now "mount" other Database instances at
certain locations.  This is used to use a new SimpleDatabase instance
for each storage mount (issued with the "mount" protocol command).
Each such instance has its own database file, stored in the directory
that is specified with the "cache_directory" option.

Helps avoid unnecessary archive scans during database update on a
fresh MPD process.

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.