Say you want OpenSSH to fail non-interactively if it can't find an ssh-agent with the key required.

Normally, it prompts to unlock the key, which in this case (script running over-night) will not work. Options such as `BatchMode' and `KbdInteractiveAuthentication' do not work.

What ended up working was setsid(1), eg.

  exec setsid --fork ssh "$@"