Lua 5.2 support, detecting definition of `unpack` function

Lua 5.2 had moved `unpack()` into `table.unpack()` (which is also noted on the reference http://www.lua.org/manual/5.2/manual.html#8.2).
Currently the official implementation of Lua 5.2 has been offering `unpack()` for the sake of compatibility, but the function is obsolete and does not exist on LuaJ.