Revamp of EOF handling in all network connections, pipes and other

data channels. Should comprehensively fix 'half-closed', in principle,
though it's a big and complicated change and so there's a good chance
I've made at least one mistake somewhere.

All connections should now be rigorous about propagating end-of-file
(or end-of-data-stream, or socket shutdown, or whatever) independently
in both directions, except in frontends with no mechanism for sending
explicit EOF (e.g. interactive terminal windows) or backends which are
basically always used for interactive sessions so it's unlikely that
an application would be depending on independent EOF (telnet, rlogin).

EOF should now never accidentally be sent while there's still buffered
data to go out before it. (May help fix 'portfwd-corrupt', and also I
noticed recently that the ssh main session channel can accidentally
have MSG_EOF sent before the output bufchain is clear, leading to
embarrassment when it subsequently does send the output).

[originally from svn r9279]

putty.h

@@ -595,6 +595,11 @@ void ldisc_update(void *frontend, int echo, int edit);
 void update_specials_menu(void *frontend);
 int from_backend(void *frontend, int is_stderr, const char *data, int len);
 int from_backend_untrusted(void *frontend, const char *data, int len);
+/* Called when the back end wants to indicate that EOF has arrived on
+ * the server-to-client stream. Returns FALSE to indicate that we
+ * intend to keep the session open in the other direction, or TRUE to
+ * indicate that if they're closing so are we. */
+int from_backend_eof(void *frontend);
 void notify_remote_exit(void *frontend);
 /* Get a sensible value for a tty mode. NULL return = don't set.
  * Otherwise, returned value should be freed by caller. */