Modeled after std::filesystem::operator/() from C++17.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

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

Replaces StatFile(), with a portable data object.

"override" implies "virtual".

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

Replaces map_to_relative_path() from Mapper.cxx.

Hide inside CreateLocalStorage().

Prepare for the plugin interface.

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