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

Wildcard matches are directly applied to all filenames in
subdirectories without any attempt at matching relative paths.

This change is based on the following feature request:

  http://bugs.musicpd.org/view.php?id=3729

std::auto_ptr is deprecated, and std::unique_ptr is much better
anyway.

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.

Replaces map_to_relative_path() from Mapper.cxx.

Prepare for the plugin interface.

Keep only a reference.

Prepare to make this a new plugin API, for example to use a SMB share
for the music_directory.

Decouples db/update/Remove.cpp from global variables.

Remove dependency on the DatabaseSimple library.

Reduce overhead.

Move global variables into the new classes.  That may allow multiple
update threads for multiple databases one day.

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