prompt: don't pass the return code to the prompt functions

Since the return code is only needed in one prompt function and it might make
sense in the future to add additional values passed to the prompt function the
return code is no longer passed as an argument to the prompt functions but in
the variable _R9E_PROMPT_RETURN_CODE.
added a helper function array_contains
prompt: don't return false from non const prompt function but register them

non const prompt functions now have to be registered explicitly. The return
value of the prompt function is no longer used to distinguish const from non
const functions. This ensures that non const prompt functions don't have to be
called during preparation of the prompt.