Improve a recent change in 'mule-cmds.el'

Improve a recent change in 'mule-cmds.el'

* lisp/international/mule-cmds.el
(select-safe-coding-system--format-list): Display codepoints in
hex and as characters.  Display the ellipsis in a better place.
Enlarge the limit to 5 codepoints, as the display is now easier to
grasp.
(select-safe-coding-system-interactively): Don't use
'select-safe-coding-system--format-list' for "rejected"
coding-systems, as those don't come with characters and positions.