Modify flycheck-find-file-in-tree to check if directory is root directory in more platform neutral way.

Add `flycheck-root-directory-p`, which determines if a directory is the root directory in a way that also works on Windows; that is, it checks to see if `file-name-directory` reports that the directory in question is its own parent.

This is in response to issue #81, where `flycheck-find-file-in-tree` would recurse indefinitely on Windows when trying to find a file that does not exist.