bin/start-ssh-agent: Don't proceed if the SSH programs aren't around.

bin/start-ssh-agent: Don't proceed if the SSH programs aren't around.

Otherwise, when I enter an schroot session, there's no `ssh-add'
installed, so the `is the agent running' test /fails/, we delete the
socket, and then fail to start a replacement agent because `ssh-agent'
doesn't exist either.  The result is that I've now lost my socket for
my (otherwise completely functional) agent, and there is sadness.