"override" implies "virtual".

Assume the configured filesystem character set is also used by the NFS
server.

Swap the order of adding the URI and the slash, because appending is
cheaper than inserting.

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.