add "primaries" as a synonym for "masters" in named.conf

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.

bin/tests/system/rrsetorder/ns2/named.conf.in

@@ -31,6 +31,6 @@ options {
 
 zone "." {
 	type slave;
-	masters { 10.53.0.1; };
+	primaries { 10.53.0.1; };
 	file "root.bk";
 };