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.

modules/clock.c

@@ -82,7 +82,7 @@ run(struct module *mod)
 static struct module *
 clock_new(struct particle *label, const char *date_format, const char *time_format)
 {
-    struct private *m = malloc(sizeof(*m));
+    struct private *m = calloc(1, sizeof(*m));
     m->label = label;
     m->date_format = strdup(date_format);
     m->time_format = strdup(time_format);