- added tdb_flags option to tdb_open()

- added tdb_flags option to tdb_open()
- added TDB_CLEAR_IF_FIRST flag to clear the database if this is the
  first attached process. Useful for non-persistent databases like our
  locking area (this will also make upgrades to new database layouts easier)
- use lock_path() in a couple of places
- leave connections database open while smbd running
- cleaned up some tdb code a little, using macros for constants