(Info-search-whitespace-regexp): Fix backslashes.

(Info-search-whitespace-regexp): Fix backslashes.
(Info-search): Add new optional arguments for the sake of isearch.
Replace whitespace in Info-search-whitespace-regexp literally.
Add backward search.  Don't call `Info-select-node' if regexp is
found in the same Info node.  Don't add node to Info-history for
wrapped isearch.
(Info-search-backward, Info-isearch-search, Info-isearch-wrap)
(Info-isearch-push-state, Info-isearch-pop-state): New funs.
(Info-mode): Set local variables `isearch-search-fun-function',
`isearch-wrap-function', `isearch-push-state-function',
`search-whitespace-regexp'.