Add a db cursor abstraction and interfaces for using it

Add a db cursor abstraction and interfaces for using it
- Callers are currently required to pass all sorts of unnecessary
  gunk to the cursor operations just for performance statistics
  etc, and the interface doesn't match the uses inside rpmdb.c
  very well. This adds a more "rpm-style" interface around the
  BDB API.
- Cursor open + close can only fail on invalid arguments (which would
  be programmer errors), and various conditions on replication which
  we dont care about. So we essentially ignore the errors, except
  for cursor open which will return NULL to indicate an error.
- As an intermediate step, these are just wrappers to the older
  dbiFoo interfaces to permit converting things to the new interfaces
  piece by piece.