Clean dangling agent socket

agent.go

@@ -85,6 +85,7 @@ func (a *Agent) start() {
 	fmt.Fprintf(os.Stderr, "Start new agent for identity %s\n", a.Identity.Name)
 	sock := a.Path + ".sock"
 
+	syscall.Unlink(sock)
 	cmd := exec.Command("ssh-agent", "-a", sock)
 	o, e, err := capture3(cmd)
 	if err != nil {