Uppity: add stunt for unauthorised agent forwarding attempts.

With the new --open-unconditional-agent-socket option, every time
Uppity receives an SSH connection, it will immediately open a Unix-
domain socket and attempt to do agent forwarding on it, in the sense
that any connection to that socket will be turned into an
"auth-agent@openssh.com" CHANNEL_OPEN request on whichever SSH
connection it was associated with.

That connection-global socket is independent of any that are created
as part of setting up a session channel. The pathname of the socket
file is written to the server's event log (there being no other
sensible place to send it).

The aim is that this allows me to test the behaviour of an SSH client
if the server tries to open an agent-forwarding channel outside the
usual context. In particular, it allows me to test the change I just
made in the previous commit, that if you enable agent forwarding in
the client configuration, then auth-agent channels opened by the
server are accepted even if no session channel opened by the client
has sent an auth-agent-req. More importantly, it allows me to check
that I _haven't_ accidentally arranged that those channels are
accepted even when agent forwarding is _not_ permitted by the client
configuration!

Implementation details: the agent forwarding socket was previously
implemented as part of the internal sesschan structure. I've moved it
out into a little sub-struct of its own which can be created
independently of a sesschan.

unix/uxserver.c

@@ -783,6 +783,8 @@ int main(int argc, char **argv)
             conf_set_int(conf, CONF_logxfovr, LGXF_OVR);
         } else if (!strcmp(arg, "--pretend-to-accept-any-pubkey")) {
             ssc.stunt_pretend_to_accept_any_pubkey = true;
+        } else if (!strcmp(arg, "--open-unconditional-agent-socket")) {
+            ssc.stunt_open_unconditional_agent_socket = true;
         } else {
             fprintf(stderr, "%s: unrecognised option '%s'\n", appname, arg);
             exit(1);