Use private environment for database rebuilds too, document

Use private environment for database rebuilds too, document

- One of the more common reasons for users to do --rebuilddb is
  a paniced environment. Throwing DB_RUNRECOVER errors at the user
  who is trying to recover by rebuilding the db isn't terribly
  productive (RhBug:590710). Use a private environment while
  rebuilding for both the original and new database, and dont bother
  with CDB which only slows things down when there are no other
  players present.
- Verify wants the same flags but for different reasons...