Merge permission for modified/unmodified & expand permission scope

I don’t see any reason why unmodified and modified require different clauses,
particularly since the first one also contemplates modifying (“modify any
unmodified Output”). The second paragraph is correct and complete for both
modified and unmodified.

The CC0 theoretically gives rights beyond propagating, conveying and
modifying. These terms would be read as a limitation on the greater rights
under the CC0, which I don't think is what's intended.

Finally, the added content may not be copyrightable, in which case they don’t
get the additional permission under the prior revision.