MEDIUM: init: use initcall for all fixed size pool creations

This commit replaces the explicit pool creation that are made in
constructors with a pool registration. Not only this simplifies the
pools declaration (it can be done on a single line after the head is
declared), but it also removes references to pools from within
constructors. The only remaining create_pool() calls are those
performed in init functions after the config is parsed, so there
is no more user of potentially uninitialized pool now.

It has been the opportunity to remove no less than 12 constructors
and 6 init functions.
diff --git a/src/checks.c b/src/checks.c
index 55efb80..282ed88 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -72,8 +72,8 @@
 static int wake_srv_chk(struct conn_stream *cs);
 static void __event_srv_chk_r(struct conn_stream *cs);
 
-static struct pool_head *pool_head_email_alert   = NULL;
-static struct pool_head *pool_head_tcpcheck_rule = NULL;
+DECLARE_STATIC_POOL(pool_head_email_alert,   "email_alert",   sizeof(struct email_alert));
+DECLARE_STATIC_POOL(pool_head_tcpcheck_rule, "tcpcheck_rule", sizeof(struct tcpcheck_rule));
 
 
 static const struct check_status check_statuses[HCHK_STATUS_SIZE] = {
@@ -3514,13 +3514,6 @@
 	return 0;
 }
 
-__attribute__((constructor))
-static void __check_init(void)
-{
-	pool_head_email_alert   = create_pool("email_alert",   sizeof(struct email_alert),   MEM_F_SHARED);
-	pool_head_tcpcheck_rule = create_pool("tcpcheck_rule", sizeof(struct tcpcheck_rule), MEM_F_SHARED);
-}
-
 REGISTER_POST_CHECK(start_checks);
 
 /*