arptables: use ->save for arptables-save, like xtables

arptables: use ->save for arptables-save, like xtables

arptables-save will show
-A OUTPUT --h-length 6 --h-type 1 -j MARK --set-xmark 0x1/0xffffffff
as
--h-length 6 --h-type Ethernet -j MARK MARK set 0x1

Because it uses ->print() instead of ->save().
Switch it to use ->save, we can then also drop special handling of
CLASSIFY target.

Signed-off-by: Florian Westphal <fw@strlen.de>