- DB: update documentation and man pages for KIWIMigrate
- DB: rebuild documentation
- add alternative 'kiwi --describe' option
  * update kiwi usage message. Don't talk about 'migration'
    Be more generic and talk about 'analyse & description'
    because that's in fact what the code does. migration is
    just one path one can perform.
- make checking for empty directories fast in KIWIMigrate