The previous commit made a bad situation worse by replacing a K&R
declaration with an incorrect prototype, thus breaking any software that
calls rl_message() and doesn't know the secret sauce to get the correct
prototype (e.g. devel/rlwrap). Fix this by dropping K&R support and
instead offering an annotated prototype for compilers that understand
the GCC format attribute and an unannotated one for those that don't.
Fixes: ee994524fffd
PR: 281633