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

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.