arptables-nft: Fix MARK target parsing and printing

arptables-nft: Fix MARK target parsing and printing

Legacy arptables parses mark values in hex no matter if prefixed with
'0x' or not. Sadly, this is not easily achievable with guided option
parser. Hence fall back to the old 'parse' callback. The introduced
target definition is valid only for revision 2, but that's consistent
with legacy arptables.

When printing, use --set-mark option instead of --set-xmark.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>