An assertion failure in UpdateQueuedSong() could trigger because the
`prev` parameter is always `nullptr`, but `queued` may be set.  And in
fact, calling UpdateQueuedSong() is only necessary when the queued
song was edited, to re-queue it with the new range.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/901

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Commit 13208bf5 added range support to
the `move` command, but applied the wrong offset to the `to` variable.
When the source range is before the current song, and the song thus
gets decremented by the range size, then the final destination offset
must also be decremented by the range size.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/663

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.

Fixes #165.

http://bugs.musicpd.org/view.php?id=4470

Manipulates the playback range of a queued song.

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

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

Make the DetachedSong(Song) conversion constructor private.  Everybody
should use map_song_detach() which will take over more
responsibilities soon.