dbwrap_tool: add option "--non-persistent" and force excatly one of "--[non-]persistent"

dbwrap_tool: add option "--non-persistent" and force excatly one of "--[non-]persistent"

We want to force users of dbwrap_tool to explicitly specify
persistent or non-persistent. Otherwise, one could easily
by accident wipe a whole database that is actually persistent
but not currently opened by a samba process, just by openeing
the DB with the default non-persistent mode...

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>