JSR $FFCF / branch based on C flag

$FFCF is a publically documented call point in the C64 KERNAL,
and indicates its response via the C flag, so such constructions
in callers are totally expected and anticipated normal use of the
API.  Since using an API is an expected use of a system, it can
be argued that there is an implicit license to do such things, in
the non-existent reality where such a sequence could be copyrighted.