arptables: Fix formatting of numeric --h-type output

arptables: Fix formatting of numeric --h-type output

Arptables expects numeric arguments to --h-type option in hexadecimal
form, even if no '0x'-prefix is present. In contrast, it prints such
values in decimal. This is not just inconsistent, but makes it
impossible to save and later restore a ruleset without fixing up the
values in between.

Assuming that the parser side can't be changed for compatibility
reasons, fix the output side instead.

This is a day 1 bug and present in legacy arptables as well, so treat
this as a "feature" of arptables-nft and omit a Fixes: tag.

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