NFLOG: remove header dependency, add a test case

This change removes detection of linux/netfilter/nfnetlink_log.h, which
was only used to provide two constants. The constants are now defined in
print-nflog.c, making it possible to compile (and test) the NFLOG
decoder on all systems, not only Linux.

There is now a test case for the decoder, it was produced on a Linux
host with:

iptables -A INPUT -p udp --source 74.82.42.42 -j NFLOG --nflog-group 20
dig @74.82.42.42 ...