all: rework fetching/generating config in cd mode

Config fetching/generating in cd mode is currently weird, error prone,
and easy for user to break ctrld when using custom config.

This commit reworks the flow:

 - Fetching config from Control D API.
 - No custom config, use the current default config.
 - If custom config presents, but there's no listener, use 0.0.0.0:53.
 - Try listening on current ip+port config, if ok, ctrld could be a
   direct listener with current setup, moving on.
 - If failed, trying 127.0.0.1:53.
 - If failed, trying current ip + port 5354
 - If still failed, pick a random ip:port pair, retry until listening ok.

With this flow, thing is more predictable/stable, and help removing the
Config interface for router.

config_test.go

@@ -1,6 +1,7 @@
 package ctrld_test
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/go-playground/validator/v10"
@@ -56,6 +57,20 @@ func TestLoadDefaultConfig(t *testing.T) {
 	assert.Len(t, cfg.Upstream, 2)
 }
 
+func TestConfigOverride(t *testing.T) {
+	v := viper.NewWithOptions(viper.KeyDelimiter("::"))
+	ctrld.InitConfig(v, "test_load_config")
+	v.SetConfigType("toml")
+	require.NoError(t, v.ReadConfig(strings.NewReader(testhelper.SampleConfigStr(t))))
+	cfg := ctrld.Config{Listener: map[string]*ctrld.ListenerConfig{
+		"0": {IP: "127.0.0.1", Port: 53},
+	}}
+	require.NoError(t, v.Unmarshal(&cfg))
+
+	assert.Equal(t, "10.10.42.69", cfg.Listener["1"].IP)
+	assert.Equal(t, 1337, cfg.Listener["1"].Port)
+}
+
 func TestConfigValidation(t *testing.T) {
 	tests := []struct {
 		name    string