new option stop_after_s

Ability to stop reading PCAP after specified number of seconds.
It depends on fact that extract-clients script renumbers packets
so first packet has timestamp 0.

Quirk:
Timestamps are checked only after they are already delayed by dnsjit
module. For this reason PCAP reading is in fact terminated when
time for the first packet _after_ the specified time limit passes.
In practice this would be noticeable only if delays between packets
were long.