add os library to iosafe

sim/iosafe.lua

@@ -5,6 +5,8 @@ local iosafe = io
 _G.io = {safe = true}
 local requiresafe = require
 _G.require = nil
+local ossafe = os
+_G.os = {safe = true}
 
 local savedir = OPT_SAVE_DIR
 
@@ -20,3 +22,5 @@ function io.open(fn, mode)
 	
 	return iosafe.open(fn, mode)
 end
+
+os.clock = ossafe.clock