s3:net_idmap_dump support dumping autorid backend

s3:net_idmap_dump support dumping autorid backend

- remember the type of idmapping database (tdb or autorid)
  this allows to make rest of the code (e.g. dump) know which database-style it will encounter
- add a seperate dump function for autorid
- default to TDB if db-file is given on the command-line

Pair-Programmed-With: Ralph Wuerthner <ralph.wuerthner@de.ibm.com>

Signed-off-by: Christian Ambach <ambi@samba.org>
Signed-off-by: Ralph Wuerthner <ralph.wuerthner@de.ibm.com>
Reviewed-by: Michael Adam <obnox@samba.org>