Make rpmdsMerge() return something meaningful

Make rpmdsMerge() return something meaningful

- The API documentation was always incorrect and meaningless, as
  rpmdsMerge() can merge any number of entries so there's no single
  index to return. Instead return the number of merged entries
  and adjust the docs to match the new behavior.
- This is of course an incompatible API change in a strict sense,
  but then since the documentation was always wrong to begin with
  anybody looking at rpmdsMerge() return is relying on undocumented
  behavior really. The compatible way to use this is to check
  for < 0 for errors and assume success otherwise.
(cherry picked from commit a48b11edbd00c4f0dbdd3462e659dd1d3bcf21ac)