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.go

@@ -12,6 +12,8 @@ import (
 	"net/url"
 	"os"
 	"runtime"
+	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -123,6 +125,25 @@ func (c *Config) HasUpstreamSendClientInfo() bool {
 	return false
 }
 
+// FirstListener returns the first listener config of current config. Listeners are sorted numerically.
+//
+// It panics if Config has no listeners configured.
+func (c *Config) FirstListener() *ListenerConfig {
+	listeners := make([]int, 0, len(c.Listener))
+	for k := range c.Listener {
+		n, err := strconv.Atoi(k)
+		if err != nil {
+			continue
+		}
+		listeners = append(listeners, n)
+	}
+	if len(listeners) == 0 {
+		panic("missing listener config")
+	}
+	sort.Ints(listeners)
+	return c.Listener[strconv.Itoa(listeners[0])]
+}
+
 // ServiceConfig specifies the general ctrld config.
 type ServiceConfig struct {
 	LogLevel              string `mapstructure:"log_level" toml:"log_level,omitempty"`