arptables: Warn when ignoring '-p' option

arptables: Warn when ignoring '-p' option

Legacy arptables has been silently ignoring this flag (plus mandatory
argument) since day 1. Retain compatibility to that behaviour but inform
users that a part of their rule does nothing.

Since arp is the only family which didn't provide a proto_parse
callback, implement one for the sole purpose of printing the warning. As
a side-effect, caller no longer has to check callback's existence.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>