dsdb: Use samdb_system_container_dn() in pdb_samba_dsdb_*()

dsdb: Use samdb_system_container_dn() in pdb_samba_dsdb_*()

This makes more calls to add children, but avoids the cn=system string in the
codebase which makes it easier to audit that this is always being built
correctly.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Mon Jul 31 07:20:21 UTC 2023 on atb-devel-224