prompt_set
rv = prompt_set(el, p, 0, op, 1);
rv = prompt_set(el, p, (wchar_t)c, op, 1);
ret = prompt_set(el, p, 0, op, 0);
ret = prompt_set(el, p, c, op, 0);
libedit_private int prompt_set(EditLine *, el_pfunc_t, wchar_t, int, int);