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().

This is what we get from the storage plugin via FileInfo.  Fixes a
compiler warning on Mac OS X where dev_t appears to be signed.

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

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.

Replaces GLib's GError.

Let the database plugin do the match.