Add primaries, parental-agents as synonyms

Add primaries, parental-agents as synonyms

Add back the top blocks 'parental-agents', 'primaries', and 'masters'
to the configuration. Do not document them as so many names for the
same clause is confusing.

This has a slight negative side effect that a top block 'primaries'
can be referred to with a zone statement 'parental-agents' for example,
but that shouldn't be a big issue.