Fixed PUSHING mode of piconfz.

Requires GETTING_MORE flag to be set, too. This mode assumes a
whole concatenation being the payload. It pushes what it gets for
the moment. Useful for fast PL and slow PS as an alternative for
setting data_size to the concatenation size while both variants
do not prepend a payload size header. Instead, data_size can be set
larger in order to push more concatenations at once.