System for adding and removing keywords.

System for adding and removing keywords.
Both local (previously added keyword) and global keywords
can be removed.
(font-lock-remove-keywords): New user-level function.
(font-lock-update-removed-keyword-alist): New internal function.
(font-lock-removed-keywords-alist): New variable.
(font-lock-add-keywords): Updates `font-lock-removed-keywords-alist'.
Empty `font-lock-keywords-alist' when `append' is `set' to avoid
growing datastructures.
(font-lock-set-defaults): Removes keywords stored in
`font-lock-removed-keywords-alist' after local keywords added.
(c-keywords, c++-keywords): Better "case" support for
complex constant expressions, e.g. "case 1<<A_BIT_NO:".
(c-keywords): Only highlight preprocessor
directives when spelled correctly.
(font-lock-match-c++-structor-declaration,
c++-keywords): Fontify constructors and destructors with function
face inside C++ class declarations.