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.

lib/dns/sdlz.c

@@ -1086,7 +1086,7 @@ modrdataset(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
 		goto cleanup;
 
 	result = dns_master_rdatasettotext(sdlznode->name, rdataset,
-					   style, buffer);
+					   style, NULL, buffer);
 	if (result != ISC_R_SUCCESS)
 		goto cleanup;