* File.expand_path: support drive letter and ALT_SEPARATOR

* File.dirname: support ALT_SEPARATOR
* File.basename: ditto.

* IO.popen: raise NotImplementedError
* IO.pipe: ditto.
* `cmd`:  ditto.
* File#flock: ditto.
* FileTest.pipe?: ditto.
* FileTest.symlink?: ditto.
* FileTest.socket?: ditto.