dns_master_indent and dns_master_indentstr must not be global

The indentation for dumping the master zone was driven by two
global variables dns_master_indent and dns_master_indentstr.  In
threaded mode, this becomes prone to data access races, so this commit
converts the global variables into a local per-context tuple that
consist of count and string.

bin/tests/system/digdelv/tests.sh

@@ -752,6 +752,7 @@ if [ -x "$DIG" ] ; then
     status=$((status+ret))
   fi
 
+  n=$((n+1))
   echo_i "check that dig +unexpected works ($n)"
   ret=0
   dig_with_opts @10.53.0.6 +unexpected a a.example > dig.out.test$n || ret=1