* lisp/replace.el (replace-lax-whitespace): New defcustom.

* lisp/replace.el (replace-lax-whitespace): New defcustom.
(query-replace, query-replace-regexp, query-replace-regexp-eval)
(replace-string, replace-regexp): Mention it in docstrings.
(perform-replace, replace-highlight): Let-bind
isearch-lax-whitespace and isearch-regexp-lax-whitespace according
to the values of replace-lax-whitespace and regexp-flag.
Don't let-bind search-whitespace-regexp.

* lisp/isearch.el (isearch-query-replace): Let-bind
replace-lax-whitespace instead of let-binding
replace-search-function and replace-re-search-function.
(isearch-lazy-highlight-search): Let-bind isearch-lax-whitespace
and isearch-regexp-lax-whitespace to lazy-highlight variables.
(isearch-toggle-symbol): Set isearch-regexp to nil
in isearch-word mode (like in isearch-toggle-word).