modules: use calloc() instead of malloc()

In cases where it makes sense, use calloc() instead of malloc():

* When allocating large objects with many members, many for which
  NULL/0 is a good default value.
* Arrays etc where we explicitly initialize to NULL anyway.

module.c

@@ -6,17 +6,9 @@
 struct module *
 module_common_new(void)
 {
-    struct module *mod = malloc(sizeof(*mod));
-    mod->bar = NULL;
+    struct module *mod = calloc(1, sizeof(*mod));
     mtx_init(&mod->lock, mtx_plain);
-    mod->private = NULL;
     mod->destroy = &module_default_destroy;
-
-    /* No defaults for these; must be provided by implementation */
-    mod->run = NULL;
-    mod->content = NULL;
-    mod->refresh_in = NULL;
-
     return mod;
 }