pktdump - add extension to packets

This is more of a quality of life thing.

Can you add the .pkt extension to bad packets that are created using something pktdump -split

07710f20.pkt.msg-hdr.bad.good

rather than

07710f20.msg-hdr.bad.good

That makes it easy to copy and paste the name when renaming and having to type the .pkt for each good package.