Lua: delete loadstring()

Fixes the vulnerability introduced in commit 52ae31efb2.

(cherry-picked from commit 6450bada57)

src/scripting/lua_kernel_base.cpp

@@ -551,6 +551,8 @@ lua_kernel_base::lua_kernel_base()
 
 	lua_pushcfunction(L, intf_load);
 	lua_setglobal(L, "load");
+	lua_pushnil(L);
+	lua_setglobal(L, "loadstring");
 
 	cmd_log_ << "Initializing package repository...\n";
 	// Create the package table.