do not use a global ifblock stack

do not use a global ifblock stack

Using a global ifblock stack for all parsed ifblock objects causes
problems when doing sub-parsing in objects taking other objects as
parameters, because the possibly non-empty stack at startup leads to
false alarm when checking for stack emptiness after parsing the objects.

Use a void ** as the object to pass around, so callers don't need to
know struct ifblock_stack_obj.