netfilter: ipt_LOG/ip6t_LOG: add option to print decoded MAC header

The LOG targets print the entire MAC header as one long string, which is not
readable very well:

IN=eth0 OUT= MAC=00:15:f2:24:91:f8:00:1b:24:dc:61:e6:08:00 ...

Add an option to decode known header formats (currently just ARPHRD_ETHER devices)
in their individual fields:

IN=eth0 OUT= MACSRC=00:1b:24:dc:61:e6 MACDST=00:15:f2:24:91:f8 MACPROTO=0800 ...
IN=eth0 OUT= MACSRC=00:1b:24:dc:61:e6 MACDST=00:15:f2:24:91:f8 MACPROTO=86dd ...

The option needs to be explicitly enabled by userspace to avoid breaking
existing parsers.

Signed-off-by: Patrick McHardy <kaber@trash.net>