require improvements, remove global Util

sys/apis/fs/ramfs.lua

@@ -1,3 +1,5 @@
+local Util   = require('util')
+
 local ramfs = { }
 
 function ramfs.mount(dir, nodeType)

sys/apis/fs/urlfs.lua

@@ -1,4 +1,5 @@
 local synchronized = require('sync')
+local Util         = require('util')
 
 local urlfs = { }