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.

ssh/ssh.c

@@ -789,7 +789,8 @@ static char *connect_to_host(
 
         ssh->s = new_connection(addr, *realhost, port,
                                 false, true, nodelay, keepalive,
-                                &ssh->plug, ssh->conf);
+                                &ssh->plug, ssh->conf,
+                                log_get_policy(ssh->logctx));
         if ((err = sk_socket_error(ssh->s)) != NULL) {
             ssh->s = NULL;
             seat_notify_remote_exit(ssh->seat);