Fixes default policy override for semantic checks.

Commit refs #736.

src/knot/conf/cf-parse.y

@@ -114,6 +114,7 @@ system:
 zone_start: TEXT {
    this_zone = malloc(sizeof(conf_zone_t));
    memset(this_zone, 0, sizeof(conf_zone_t));
+   this_zone->enable_checks = -1; // Default policy applies
    this_zone->name = $1;
 
    // Append mising dot to ensure FQDN

src/knot/conf/conf.c

@@ -217,8 +217,8 @@ static int conf_process(conf_t *conf)
 		conf_zone_t *zone = (conf_zone_t*)n;
 
 		// Default policy for semantic checks
-		if (conf->zone_checks) {
-			zone->enable_checks = 1;
+		if (zone->enable_checks < 0) {
+			zone->enable_checks = conf->zone_checks;
 		}
 
 		// Normalize zone filename