C++: fontify identifier in declaration following "public:" correctly.

C++: fontify identifier in declaration following "public:" correctly.
* progmodes/cc-langs.el (c-decl-start-colon-kwd-re): New lang var
to match "public", etc.
(c-decl-prefix-re): Add ":" into the C++ value.
* progmodes/cc-engine.el (c-find-decl-prefix-search): Refactor a
bit.  Add a check for a ":" preceded by "public", etc.