Blender Git Loki

Cleanup: readfile.c: use id_us_ensure_real() instead of own custom 'at least one user' handling.

Note that in master this is mere cleanup, which shall not change much things
(aside from FAKE_USER now being taken into account here too), and mostly
makes things more consistent, but in id-remap branch this becomes mandatory
due to better handling of this ugly 'ensure_real' stuff re user count.