path: make format() consistent and more functional

Make the win32 and posix versions of path.format() consistent in when
they add a directory separator between the dir and base parts of the
path (always add it unless the dir part is the same as the root).

Also, path.format() is now more functional in that it uses the name
and ext parts of the path if the base part is left out and it uses
the root part if the dir part is left out.

Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2408