This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

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

Empty currently, but code will be added song.

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.

Don't use global variable.

Works around a build failure on Solaris because annoyingly, Solaris
reserves the name "queue".  This rename was pending anyway.

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.