netfilter: Introduce NFPROTO_* constants

The netfilter subsystem only supports a handful of protocols (much
less than PF_*) and even non-PF protocols like ARP and
pseudo-protocols like PF_BRIDGE. By creating NFPROTO_*, we can earn a
few memory savings on arrays that previously were always PF_MAX-sized
and keep the pseudo-protocols to ourselves.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>