* lisp/international/mule-cmds.el (universal-coding-system-argument): Rewrite

* lisp/international/mule-cmds.el (universal-coding-system-argument): Rewrite

Use the new `prefix-command-*` hooks and functions so it interacts
better with other prefix commands (and with itself), and so the
pre/post-command-hook and other command-loop operations are performed
"normally".

(mule-cmds--prefixed-command-next-coding-system)
(mule-cmds--prefixed-command-last-coding-system): New vars.
(mule-cmds--prefixed-command-pch, mule-cmds--prefixed-command-echo)
(mule-cmds--prefixed-command-preserve): New functions.