box: Add a precaution about undefined behavior of XXXX_BOX_TYPE_YYYY.user.id .
box: Use compound literal to convert 4-byte integer into NULL-terminated string.
box: Make consistent with return type of box constructor functions.
box: Rename CHECK_LARGESIZE into UPDATE_BOX_SIZE_COMMON_PROCESS.

The previous name of the macro is improper.
Implementation of referencing external media files.