MEDIUM: init: convert all trivial registration calls to initcalls

This switches explicit calls to various trivial registration methods for
keywords, muxes or protocols from constructors to INITCALL1 at stage
STG_REGISTER. All these calls have in common to consume a single pointer
and return void. Doing this removes 26 constructors. The following calls
were addressed :

- acl_register_keywords
- bind_register_keywords
- cfg_register_keywords
- cli_register_kw
- flt_register_keywords
- http_req_keywords_register
- http_res_keywords_register
- protocol_register
- register_mux_proto
- sample_register_convs
- sample_register_fetches
- srv_register_keywords
- tcp_req_conn_keywords_register
- tcp_req_cont_keywords_register
- tcp_req_sess_keywords_register
- tcp_res_cont_keywords_register
- flt_register_keywords
diff --git a/src/connection.c b/src/connection.c
index 5022772..6214775 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -14,6 +14,7 @@
 
 #include <common/compat.h>
 #include <common/config.h>
+#include <common/initcall.h>
 #include <common/namespace.h>
 #include <common/hash.h>
 #include <common/net_helper.h>
@@ -1314,9 +1315,4 @@
 	{ /* END */ },
 }};
 
-
-__attribute__((constructor))
-static void __connection_init(void)
-{
-	sample_register_fetches(&sample_fetch_keywords);
-}
+INITCALL1(STG_REGISTER, sample_register_fetches, &sample_fetch_keywords);