path: fix path.relative UNC path result

When the result of a path.relative() is an absolute UNC path, it should
include the leading backslashes.

Fixes: https://github.com/nodejs/node/issues/8444
PR-URL: https://github.com/nodejs/node/pull/8523


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>