r20871: implement the validFSMOs constructed attribute on the rootdse

r20871: implement the validFSMOs constructed attribute on the rootdse
for the schema, domain naming and pdc fsmo roles

infrastructure and rid manager will be added later,
when we have module for them

metze
(This used to be commit 308f9cf822a3a34dae28a5fa5aa850e2adbeb472)