CLEANUP: vars: move the per-process variables initialization to vars.c

There's no point keeping the vars_init_head() call in init() when we
already have a vars_init() registered at the right time to do that,
and it complexifies the boot sequence, so let's move it there.
diff --git a/src/haproxy.c b/src/haproxy.c
index 6f591d6..4d95ae6 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -1540,9 +1540,6 @@
 	/* Initialise lua. */
 	hlua_init();
 
-	/* Initialize process vars */
-	vars_init_head(&proc_vars, SCOPE_PROC);
-
 	global.tune.options |= GTUNE_USE_SELECT;  /* select() is always available */
 #if defined(USE_POLL)
 	global.tune.options |= GTUNE_USE_POLL;
diff --git a/src/vars.c b/src/vars.c
index 4ed3a1d..e620acf 100644
--- a/src/vars.c
+++ b/src/vars.c
@@ -1335,6 +1335,8 @@
 static void vars_init()
 {
 	var_name_hash_seed = ha_random64();
+	/* Initialize process vars */
+	vars_init_head(&proc_vars, SCOPE_PROC);
 }
 
 INITCALL0(STG_PREPARE, vars_init);