readfile.c's versioning function was only used for 2 variables.
Move versioning into versioning_userdef.c so everything
is done in one function.
Note: DNA_struct_elem_find checks have been replaced with checks for
the next released version.
This is harmless, as only old preferences saved between releases can
have their values overwritten.
Note: userdef versioning should be called from `do_versions_userdef`,
this will be done separately.
15a9579a03