netsniff-ng: also support tcpdump-like filters

This is done either via:

  - netsniff-ng -i eth0 tcp and port 80
  - netsniff-ng -i eth0 -f "tcp and port 80" -s -o dump.pcap

We need to bind against libpcap from now on, but only to use its filter
compiler, no need to reinvent the wheel. Also, people are familiar with
the tcpdump syntax expression.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>