Timestamp bounds

Hi, 

https://github.com/LibtraceTeam/libtrace/blob/master/tools/tracesplit/tracesplit.c#L198

Does it make sense to set the equality check to `>=`? 

Please correct me if I'm wrong here, but since `-s` and `-e` are defined as integers, covering a range of intervals will either result in packets potentially being missed or packets being double counted.

