Change the primaries configuration objects to the more generic
remote-servers, that we can reuse for other purposes (such as
parental-agents).
(manually picked from commit 39a961112f)
as "type primary" is preferred over "type master" now, it makes
sense to make "primaries" available as a synonym too.
added a correctness check to ensure "primaries" and "masters"
cannot both be used in the same zone.
(cherry picked from commit 16e14353b1)
