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/cache.c b/src/cache.c
index acab99d..7f1de9f 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -1215,7 +1215,9 @@
 __attribute__((constructor))
 static void __cache_init(void)
 {
-	cfg_register_section("cache", cfg_parse_cache, cfg_post_parse_section_cache);
-	cfg_register_postparser("cache", cfg_cache_postparser);
 	pool_head_cache_st = create_pool("cache_st", sizeof(struct cache_st), MEM_F_SHARED);
 }
+
+/* config parsers for this section */
+REGISTER_CONFIG_SECTION("cache", cfg_parse_cache, cfg_post_parse_section_cache);
+REGISTER_CONFIG_POSTPARSER("cache", cfg_cache_postparser);