Do not disable SSH agent

[vincentbernat]

The current logic around the SSH agent is flawed and tentatives to fix it to cover more cases only leads to more complexity.

As seen in #648, to this day, nobody really knows why we disable the agent. In the meantime, many valid use cases just don't work because of disabling the agent, including the good practice to use encrypted private key files. I think it is time to bite the bullet and just leave the agent always on: "ssh" from OpenSSH does not have an option to disable the agent and nobody complains about this.