Only handle ".." and '..' quoting in shell-mode.

Only handle ".." and '..' quoting in shell-mode.
* lisp/shell.el (shell--unquote&requote-argument, shell--unquote-argument)
(shell--requote-argument): New functions.
(shell-completion-vars): Use them.
(shell--parse-pcomplete-arguments): Rename from
shell-parse-pcomplete-arguments.
* lisp/comint.el (comint-word): Obey comint-file-name-quote-list.  Simplify.
(comint--unquote&requote-argument): Don't handle ".." and '..' quoting.
Obey comint-file-name-quote-list.

Fixes: debbugs:11466