Add capability to read table flag fields from Lua API

src/script/common/c_content.h

@@ -123,6 +123,9 @@ u32                getflagsfield             (lua_State *L, int table,
                                               const char *fieldname,
                                               FlagDesc *flagdesc, u32 *flagmask);
 
+u32                read_flags_table          (lua_State *L, int table,
+                                              FlagDesc *flagdesc, u32 *flagmask);
+
 void               push_items                (lua_State *L,
                                               const std::vector<ItemStack> &items);