tdb: adding readonly locks mode to tdbbackup tool

tdb: adding readonly locks mode to tdbbackup tool

The netcmd 'domain backup offline' command will use the tdbbackup tool but
require readonly locking of tdb databases, otherwise all database access would
be blocked during a backup.  This patch adds the option.  A backup script
should use this tool with the readonly locks option after taking a transaction
lock on the target database.

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>