Fix redirections with execute. Redirection consumed the remembered

Fix redirections with execute.  Redirection consumed the remembered
input, and execute couldn't find it and mistakenly raised the
tampering deity alarm.  Closes #804644:
(saved_bytes): New.
(save_input): Set it.
(forget_input): New.
(seen_input): Don't discard, return a value for forget_input().
(save_input): Return a value for forget_input(), just because it makes
sense.
(input_to_forget, redir_authorized, prompt): Save value of
seen_input() to new input_to_forget in redir_authorized(), pass it to
forget_input() in prompt().