Add an interactive anti-spoofing prompt in Plink.

At the point when we change over the seat's trust status to untrusted
for the last time, to finish authentication, Plink will now present a
final interactive prompt saying 'Press Return to begin session'. This
is a hint that anything after that that resembles an auth prompt
should be treated with suspicion, because _PuTTY_ thinks it's finished
authenticating.

This is of course an annoying inconvenience for interactive users, so
I've tried to reduce its impact as much as I can. It doesn't happen in
GUI PuTTY at all (because the trust sigil system is used instead); it
doesn't happen if you use plink -batch (because then the user already
knows that they _never_ expect an interactive prompt); and it doesn't
happen if Plink's standard input is being redirected from anywhere
other than the terminal / console (because then it would be pointless
for the server to try to scam passphrases out of the user anyway,
since the user isn't in a position to enter one in response to a spoof
prompt). So it should only happen to people who are using Plink in a
terminal for interactive login purposes, and that's not _really_ what
I ever intended Plink to be used for (which is why it's never had any
out-of-band control UI like OpenSSH's ~ system).

If anyone _still_ doesn't like this new prompt, it can also be turned
off using the new -no-antispoof flag, if the user is willing to
knowingly assume the risk.

unix/uxcons.c

@@ -414,8 +414,26 @@ static int console_askappend(LogPolicy *lp, Filename *filename,
 	return 0;
 }
 
+bool console_antispoof_prompt = true;
 bool console_set_trust_status(Seat *seat, bool trusted)
 {
+    if (console_batch_mode || !is_interactive() || !console_antispoof_prompt) {
+        /*
+         * In batch mode, we don't need to worry about the server
+         * mimicking our interactive authentication, because the user
+         * already knows not to expect any.
+         *
+         * If standard input isn't connected to a terminal, likewise,
+         * because even if the server did send a spoof authentication
+         * prompt, the user couldn't respond to it via the terminal
+         * anyway.
+         *
+         * We also vacuously return success if the user has purposely
+         * disabled the antispoof prompt.
+         */
+        return true;
+    }
+
     return false;
 }