cmd+loopd: new cmd/loopd main method replacing the one now in ./loopd

This allows loopd to be used as a library by importing it and calling
Start.

cmd/loopd/main.go

@@ -0,0 +1,14 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/lightninglabs/loop/loopd"
+)
+
+func main() {
+	err := loopd.Start()
+	if err != nil {
+		fmt.Println(err)
+	}
+}

loopd/start.go

@@ -26,14 +26,7 @@ var (
 	swapsLock        sync.Mutex
 )
 
-func main() {
-	err := start()
-	if err != nil {
-		fmt.Println(err)
-	}
-}
-
-func start() error {
+func Start() error {
 	config := defaultConfig
 
 	// Parse command line flags.