Fix refcount logic & duplicate unhook checking.

Fix refcount logic & duplicate unhook checking.

1. LuaFuncKey() now increase the ref count when asked to, but never decrease
   the ref count. It may return LUA_UNREF if an entry is not found but asked
   not to add it.
2. LuaCheckHandler never decreases ref count and record function name only
   for mapped entry.
3. Require to provide the function name on unhook, if the function could not
   be resolved to key, do not proceed.