Make quoting of shell arguments in grep.el more robust

Make quoting of shell arguments in grep.el more robust

* lisp/progmodes/grep.el (grep-apply-setting)
(grep-compute-defaults): Adapt docstring.
(grep-quoting-style): New variable.
(grep-compute-defaults, grep-default-command)
(grep-expand-keywords, lgrep, rgrep-default-command): Use it.