If `SimpleDatabase::Visit` is called on a database that contains a mounted directry the URIs of the elements passed to the callbacks are not prefixed by the mountpoint path. This leads to lsinfo and add not working because they use the wrong URI. This pull request is using the `WalkMount` helper function to create prefixed versions of `VisitDirectory`, `VisitSong` and `VisitPlaylist` to add the correct prefix to the parameters of the callback functions.

Signed-off-by: FlashSystems <developer@flashsystems.de>

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

Implementations shall use exceptions instead.

Check for 0 in "device" and "inode" instead.

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.

Move to SimpleDatabase::GetSong() to give that method more control.

Code can now be reused in LookupSong().

Decouple command/OtherCommands.cxx from the "simple" database plugin.

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

Using a variable-size struct with embedded string is not worth the
trouble here.  There are not so many Directory objects.

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.

Explicit attribute initialization.