ido, iswitchb, icomplete: Use `C-.' and `C-,' for cycling (Bug#13708)

* lisp/icomplete.el (icomplete-minibuffer-map): Unbind `C-s' and `C-r'.
Bind `C-.' and `C-,' to `icomplete-forward-completions' and
`icomplete-backward-completions'.

* lisp/ido.el (ido-init-completion-maps): Bind `C-.' and `C-,' to
`ido-next-match' and `ido-prev-match' resply.

* lisp/iswitchb.el (iswitchb-read-buffer): Bind `C-.' and `C-,' to
`iswitchb-next-match' and `iswitchb-prev-match' resply.