xtables-arp: Don't use ARPT_INV_*

xtables-arp: Don't use ARPT_INV_*

Arptables invflags are partly identical to IPT_INV_* ones but the bits
are differently assigned. Eliminate this incompatibility by definition
of the unique invflags in nft-arp.h on bits that don't collide with
IPT_INV_* ones, then use those in combination with IPT_INV_* ones in
arptables-specific code.

Note that ARPT_INV_ARPPRO is replaced by IPT_INV_PROTO although these
are in fact different options - yet since '-p' option is not supported
by arptables, this does not lead to a collision.

Signed-off-by: Phil Sutter <phil@nwl.cc>