Add support for ssh-agent

This allows GIT over SSH to work, including when using a container
runtime.

* If the user already has an ssh-agent process running (indicated
  by the SSH_AUTH_SOCK environment variable being set), map this socket
  into the container.

* If the user does not have an ssh-agent process running, or wishes to
  expose only a subset of their keys to the container, a new --ssh-agent
  option causes shrinkwrap to start an ssh-agent subprocess, and to add
  specified keys. As above, the socket on which the ssh-agent is
  listening is mapped into the container.

Signed-off-by: Gareth Stockwell <gareth.stockwell@arm.com>