Serial calculations do not handle wrapping

DNS serials wrap around 2^32, with the previous 2^31 values being "before" and the next 2^31 being "after". The serial calculation code needs to handle this properly when comparing two serials to determine which one is >= the other, and also when incrementing it.

The "unixtime" serial just needs to be calculated MOD 2^32, and "datecounter" requires no change.