Tunnel mode selection feature for agent mode.

[stoktamisoglu]

With this change ENABLE_GO_IOS_AGENT can be set to "user" or "kernel" to start selective mode tunnel.

[shamanec]

lgtm, but maybe @danielpaulus or @dmissmann should approve as well

[stoktamisoglu]

Hi @danielpaulus , @dmissmann , would you please look at it at your convivences? I'll submit another PR which is ready for the issue #543 . Thx.

[dmissmann]

lets verify that we get one of the expected parameters, otherwise it would panic

Suggested change

	var cmd *exec.Cmd
	if mode == "kernel" {
	cmd = exec.Command(ex, append([]string{"tunnel", "start"}, args...)...)
	}
	if mode == "user" {
	cmd = exec.Command(ex, append([]string{"tunnel", "start", "--userspace"}, args...)...)
	}
	var cmd *exec.Cmd
	switch mode {
	case "kernel":
	cmd = exec.Command(ex, append([]string{"tunnel", "start"}, args...)...)
	case "user":
	cmd = exec.Command(ex, append([]string{"tunnel", "start", "--userspace"}, args...)...)
	default:
	return fmt.Errorf("RunAgent: unknown mode: %s. Only 'kernel' and 'user' are supported", mode)
	}

[stoktamisoglu]

I've modified with the suggestion. Initial code implemented like the old code for checking only 'yes' value and if only for that value it was calling RunAgent(). Similar, initial code was calling RunAgent() only for 'kernel' and 'user' values. However what @dmissmann suggested is better practice to handle panic independent of the caller. Modified accordingly.