Fix #1285 - __builtin___snprintf_chk warning. Tighten test flags and refactor the code to squash all warnings

src/timer.c

@@ -6,8 +6,8 @@
 
 struct mg_timer *g_timers;
 
-void mg_timer_init(struct mg_timer *t, int ms, int flags, void (*fn)(void *),
-                   void *arg) {
+void mg_timer_init(struct mg_timer *t, unsigned long ms, unsigned flags,
+                   void (*fn)(void *), void *arg) {
   struct mg_timer tmp = {ms, flags, fn, arg, 0UL, g_timers};
   *t = tmp;
   g_timers = t;