netfilter: add generic ARP packet logger

This adds the generic plain text packet loggger for ARP packets. It is
based on the ebt_log code. Nevertheless, the output has been modified
to make it consistent with the original xt_LOG output.

This is an example output:

IN=wlan0 OUT= ARP HTYPE=1 PTYPE=0x0800 OPCODE=2 MACSRC=00:ab:12:34:55:63 IPSRC=192.168.10.1 MACDST=80:09:12:70:4f:50 IPDST=192.168.10.150

This patch enables packet logging from ARP chains, eg.

  nft add rule arp filter input log prefix "input: "

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>