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

(cherry picked from commit 5571ce9619d856d3c9545099366f4e0259aee8ef)

RN: A second container with name CN=System would disable the operation
 of the Samba AD DC.  Samba now finds the CN=System container by exact
 DN and not a search.

Autobuild-User(v4-18-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-18-test): Tue Aug  1 10:57:04 UTC 2023 on atb-devel-224