sshproxy: share the caller's LogPolicy.

Now new_connection() takes an optional LogPolicy * argument, and
passes it on to the SshProxy setup. This means that SshProxy's
implementation of the LogPolicy trait can answer queries like
askappend() and logging_error() by passing them on to the same
LogPolicy used by the main backend.

Not all callers of new_connection have a LogPolicy, so we still have
to fall back to the previous conservative default behaviour if
SshProxy doesn't have a LogPolicy it can ask.

The main backend implementations didn't _quite_ have access to a
LogPolicy already, but they do have a LogContext, which has a
LogPolicy vtable pointer inside it; so I've added a query function
log_get_policy() which allows them to extract that pointer to pass to
new_connection.

This is the first step of fixing the non-interactivity limitations of
SshProxy. But it's also the easiest step: the next ones will be more
involved.

otherbackends/telnet.c

@@ -739,7 +739,8 @@ static char *telnet_init(const BackendVtable *vt, Seat *seat,
      * Open socket.
      */
     telnet->s = new_connection(addr, *realhost, port, false, true, nodelay,
-                               keepalive, &telnet->plug, telnet->conf);
+                               keepalive, &telnet->plug, telnet->conf,
+                               log_get_policy(logctx));
     if ((err = sk_socket_error(telnet->s)) != NULL)
         return dupstr(err);