Recalculate cksum is now the default.
This behaviour change has been done to facilitate rewriting packet
fields post PCAP import using the new Find & Replace feature. Without
this any change in fields may cause incorrect checksums.
The earlier rationale for retaining the checksums in the PCAP file
during import was to ensure replayed packet was same as the one in the
PCAP file.
User now has a choice with this option.
