tree-wide: drop acquire_data_fd_full() helper

tree-wide: drop acquire_data_fd_full() helper

Let's drop support systems lacking memfds, i.e. pre kernel 3.17 systems.
This allows us to drastically simplify the "data fd" concept, so far
that we can remove it entirely.

This replaces acquire_data_fd() with a specialized call to
memfd_new_and_seal(), not that memfds can be the only implementation of
the concept.