journal-file: rename return parameters to ret_xyz

journal-file: rename return parameters to ret_xyz

Let's clean this up a bit, following our usual nomenclature to name
return parameters ret-xyz.

This is mostly a bit of renaming, but there's also some minor other
changes: if we return a pointer to a mmap'ed object plus its offset, in
almost all cases we are happy if either parameter is NULL in case the
caller is not interested in it. Let's fix the remaining case to do this
too, to minimize surprises.