Fix a potential time-wraparound issue in pinger.c.

A compiler warning drew my attention to the fact that 'next' in
pinger_schedule() was an int, not the unsigned long it should have
been. And looking at the code that handles it, it was also taking no
care with integer wraparound when checking whether an existing
scheduled ping should be moved forward.

So now I do something a bit more robust, by remembering what time it
_was_ when we set pinger->next, and checking if the new time value
falls in the interval between those two times.

putty.h

@@ -1450,6 +1450,7 @@ unsigned long schedule_timer(int ticks, timer_fn_t fn, void *ctx);
 void expire_timer_context(void *ctx);
 int run_timers(unsigned long now, unsigned long *next);
 void timer_change_notify(unsigned long next);
+unsigned long timing_last_clock(void);
 
 /*
  * Exports from callback.c.