Handle non-readable directories.

Handle non-readable directories.
(ido-decorations): Add 9th element for non-readable directory.
(ido-directory-nonreadable): New dynamic var.
(ido-set-current-directory): Set it.
(ido-read-buffer, ido-file-internal):
(ido-read-file-name, ido-read-directory-name): Let-bind it.
(ido-file-name-all-completions1): Return empty list for
non-readable directory.
(ido-exhibit): Print [Not readable] if directory is not readable.
(ido-expand-directory): New defun (based on tiny fix from Karl Chen).
(ido-read-file-name, ido-file-internal, ido-read-directory-name):
Use it.