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]

windows/window.c

@@ -5612,6 +5612,11 @@ int from_backend_untrusted(void *frontend, const char *data, int len)
     return term_data_untrusted(term, data, len);
 }
 
+int from_backend_eof(void *frontend)
+{
+    return TRUE;   /* do respond to incoming EOF with outgoing */
+}
+
 int get_userpass_input(prompts_t *p, unsigned char *in, int inlen)
 {
     int ret;