ssh: Set default SSH_AGENT_PID for grep

* If `SSH_AGENT_PID` is unset, grep will succeed if another ssh-agent is
  running as it will match the string ` ssh-agent` where the PID was an
  empty string.
* Set a default value for the grep to a value that will never match if
  unset, i.e. -1.
Merge branch 'ssh_agent_pid'