CC Mode: Eliminate duplicate function c-list-of-strings

CC Mode: Eliminate duplicate function c-list-of-strings

Replace it with the existing c-string-list-p.  Also put an autoload cookie in
front of c-string-list-p so that it will not be signalled as undefined by
loaddefs.el.

lisp/progmodes/cc-vars.el (c-string-list-p): Make this autoload.
(c-list-of-strings): Remove.
(c-font-lock-extra-types, c++-font-lock-extra-types)
(objc-font-lock-extra-types, java-font-lock-extra-types)
(idl-font-lock-extra-types, pike-font-lock-extra-types): In the autoload
cookies, replace c-list-of-strings with c-string-list-p.