Blender Git Loki

Cleanup: rename main preferences versioning function

BLO_version_defaults_userpref_blend -> blo_do_versions_userdef

The name was misleading as it was declared along with
BLO_update_defaults_startup_blend making it seem these functions were
related.

In fact preference defaults don't need to be updated as is done for
startup.blend since an in-memory blend file isn't used.

Rename the function to match other versioning functions
called from readfile.c. Also add/update comments on these differences.