Port File.realpath specs to File.realdirpath with adapted
Fix File.realdirpath

File.realdirpath is a similar method of File.realpath without exceptions.
It returns a fullpath of the argument. Not for dirname.
Implement Kernel#__dir__
Untag passing Kernel#__dir__ specs
Merge pull request #2817 from kachick/realdirpath-and-__dir__

Fix File.realdirpath and Implement Kernel.#__dir__