This is the backend for the upcoming "mount" command.

Build the UTF-8 version of the path automatically in the constructor.

Replaces map_to_relative_path() from Mapper.cxx.

Hide inside CreateLocalStorage().

libsmbclient is not thread-safe nor reentrant.  We must protect all
function calls with a global mutex, unfortunately.

Prepare for the plugin interface.

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