Make ordinary isearch obey search-whitespace-regexp too.

Make ordinary isearch obey search-whitespace-regexp too.

* lisp/isearch.el (search-whitespace-regexp): Make string and nil
values apply to both ordinary and regexp search.  Allow a cons
cell value to distinguish between the two.
(isearch-whitespace-regexp, isearch-search-forward)
(isearch-search-backward): New functions.
(isearch-occur, isearch-search-fun-default, isearch-search)
(isearch-lazy-highlight-new-loop): Use them.
(isearch-forward, isearch-forward-regexp): Doc fix.