Rename CUSTOM_PRINTF to ATTR_PRINTF.•••ATTR_* matches the naming scheme which will be used for function
attributes from now on.
Also, redocument it to have a better description and allow Doxygen
to parse it.
Make packing PTR queries a bit more robust.•••There is no first party modules which could be compromised before
this check was added but its better to be safe wherever possible.