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/http_conv.c b/src/http_conv.c
index c6cfdab..93b748c 100644
--- a/src/http_conv.c
+++ b/src/http_conv.c
@@ -21,6 +21,7 @@
 #include <common/config.h>
 #include <common/debug.h>
 #include <common/http.h>
+#include <common/initcall.h>
 #include <common/memory.h>
 #include <common/standard.h>
 #include <common/version.h>
@@ -335,11 +336,7 @@
 	{ NULL, NULL, 0, 0, 0 },
 }};
 
-__attribute__((constructor))
-static void __http_conv_init(void)
-{
-	sample_register_convs(&sample_conv_kws);
-}
+INITCALL1(STG_REGISTER, sample_register_convs, &sample_conv_kws);
 
 /*
  * Local variables: