Add +purgecaches startup flag. Process startup flags in :restart. Don't purge caches on :rehash.

common/modules/config.jsm

@@ -17,6 +17,7 @@ this.lazyRequire("addons", ["AddonManager"]);
 this.lazyRequire("cache", ["cache"]);
 this.lazyRequire("highlight", ["highlight"]);
 this.lazyRequire("messages", ["_"]);
+this.lazyRequire("prefs", ["localPrefs", "prefs"]);
 
 function AboutHandler() {}
 AboutHandler.prototype = {
@@ -66,6 +67,8 @@ var ConfigBase = Class("ConfigBase", {
         });
     },
 
+    get prefs() localPrefs,
+
     get has() Set.has(this.features),
 
     configFiles: [