Refactor config commit stuff to support restartless updates better

Includes restartless updates of the GUI settings (listening port etc) as
a proof of concept.

internal/config/debug.go

@@ -0,0 +1,19 @@
+// Copyright (C) 2015 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package config
+
+import (
+	"os"
+	"strings"
+
+	"github.com/calmh/logger"
+)
+
+var (
+	debug = strings.Contains(os.Getenv("STTRACE"), "config") || os.Getenv("STTRACE") == "all"
+	l     = logger.DefaultLogger
+)