Remove accidental changes of March 4. Fix backing

Remove accidental changes of March 4.  Fix backing
up when a regexp isearch is made more general.  Use symbolic
accessor functions for isearch stack frames to make usage clearer.
(search-whitespace-regexp): Made groups in documentation shy (as
is the group in the default value).
(isearch-fallback): New function, addresses problems with regexps
liberalized by `\|', adds support for liberalization by `\}' (the
general repetition construct), and incorporates behavior for
`*'/`?'.
(isearch-}-char): New command, calls `isearch-fallback' with
arguments appropriate to a typed `}'.
(isearch-*-char, isearch-|-char): Now just call `isearch-fallback'
appropriately.
(isearch-mode-map): Bind `}' to `isearch-}-char'.
(isearch-string, isearch-message,string, isearch-point,
isearch-success, isearch-forward-flag, isearch-other-end,
isearch-word, isearch-invalid-regexp, isearch-wrapped,
isearch-barrier, isearch-within-brackets,
isearch-case-fold-search):
New inline functions to read fields of a stack frame.