Introduce mg_timer_expired(). Adopt DHCP and link status to use it

src/timer.h

@@ -5,7 +5,6 @@
 struct mg_timer {
   unsigned long id;         // Timer ID
   uint64_t period_ms;       // Timer period in milliseconds
-  uint64_t prev_ms;         // Timestamp of a previous poll
   uint64_t expire;          // Expiration timestamp in milliseconds
   unsigned flags;           // Possible flags values below
 #define MG_TIMER_ONCE 0     // Call function once
@@ -21,3 +20,4 @@ void mg_timer_init(struct mg_timer **head, struct mg_timer *timer,
                    void *arg);
 void mg_timer_free(struct mg_timer **head, struct mg_timer *);
 void mg_timer_poll(struct mg_timer **head, uint64_t new_ms);
+bool mg_timer_expired(uint64_t *expiration, uint64_t period, uint64_t now);