MINOR: initcall: use initcalls for section parsers

The two calls to cfg_register_section() and cfg_register_postparser()
are now supported by initcalls. This allowed to remove two other
constructors.
diff --git a/src/dns.c b/src/dns.c
index 255f15a..2aac74f 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -2058,8 +2058,7 @@
 {
 	dns_answer_item_pool = create_pool("dns_answer_item", sizeof(struct dns_answer_item), MEM_F_SHARED);
 	dns_resolution_pool  = create_pool("dns_resolution",  sizeof(struct dns_resolution),  MEM_F_SHARED);
-
-	cfg_register_postparser("dns runtime resolver", dns_finalize_config);
 }
 
 REGISTER_POST_DEINIT(dns_deinit);
+REGISTER_CONFIG_POSTPARSER("dns runtime resolver", dns_finalize_config);