changesets: Reviewed and fixed.

- Unified parameter names.
- Added some missing allocation checks.
- Fixed changesets_empty() (was giving wrong answer if there were
  more changesets and the last one was empty).
- add_soa() now returns error code. Neede for parameter checking.
- changeset_merge() should not change the second changeset.
- Removed unused changesets_clear().
- Removed unused flags from changesets_t.
- Fixed doxygen comments.