MEDIUM: initcall: use initcalls for a few initialization functions

signal_init(), init_log(), init_stream(), and init_task() all used to
only preset some values and lists. This needs to be done very early to
provide a reliable interface to all other users. The calls used to be
explicit in haproxy.c:init(). Now they're placed in initcalls at the
STG_PREPARE stage. The functions are not exported anymore.
diff --git a/src/task.c b/src/task.c
index aeb3e4e..1f09f13 100644
--- a/src/task.c
+++ b/src/task.c
@@ -469,8 +469,8 @@
 	}
 }
 
-/* perform minimal intializations, report 0 in case of error, 1 if OK. */
-int init_task()
+/* perform minimal intializations */
+static void init_task()
 {
 	int i;
 
@@ -482,9 +482,10 @@
 	for (i = 0; i < MAX_THREADS; i++) {
 		LIST_INIT(&task_per_thread[i].task_list);
 	}
-	return 1;
 }
 
+INITCALL0(STG_PREPARE, init_task);
+
 /*
  * Local variables:
  *  c-indent-level: 8