FUEL: New option to make listener's prompt non-readonly.

FUEL: New option to make listener's prompt non-readonly.

By setting fuel-listener-prompt-read-only-p to nil, you'll be able to
delete (portions of) the prompt in the listener. This doesn't have
much value by itself, except that, moreover, a non-read-only prompt
makes some i/o operations work. E.g.:

 "input x:" print flush readln

will work with the new variable set to nil.