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/flt_trace.c b/src/flt_trace.c
index 6b65fe9..d5a4d93 100644
--- a/src/flt_trace.c
+++ b/src/flt_trace.c
@@ -12,10 +12,11 @@
 
 #include <ctype.h>
 
+#include <common/hathreads.h>
+#include <common/initcall.h>
 #include <common/standard.h>
 #include <common/time.h>
 #include <common/tools.h>
-#include <common/hathreads.h>
 
 #include <types/channel.h>
 #include <types/filters.h>
@@ -628,9 +629,4 @@
 	}
 };
 
-__attribute__((constructor))
-static void
-__flt_trace_init(void)
-{
-	flt_register_keywords(&flt_kws);
-}
+INITCALL1(STG_REGISTER, flt_register_keywords, &flt_kws);