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.

ssh2connection-server.c

@@ -297,3 +297,8 @@ void ssh2channel_send_terminal_size_change(SshChannel *sc, int w, int h)
 {
     unreachable("Should never be called in the server");
 }
+
+bool ssh2_connection_need_antispoof_prompt(struct ssh2_connection_state *s)
+{
+    return false;
+}