Add --skip option to skip some packets before writing or printing

Add --skip option to skip some packets before writing or printing

With this change, we can write/print some contiguous packets from a file.
We can also skip some packets doing a live capture.

The '--skip 0' option is allowed to help some loop in a shell script.

Examples:

Skip 3 packets when printing:
tcpdump -#n --skip 3 -r in.pcap

Write the sixth packet, if any:
tcpdump --skip 5 -c 1 -r in.pcap -w out.pcap

Write up to 5 packets after skipping 3:
tcpdump --skip 3 -c 5 -r in.pcap -w out.pcap