xtables-arp: Integrate OPT_* defines into xshared.h

xtables-arp: Integrate OPT_* defines into xshared.h

These defines are internal use only, so their actual value doesn't
matter as long as they're unique and inverse_for_options array items
match:

When negating a given option, the corresponding OPT_* value's bit is
used as an index into inverse_for_options to retrieve the corresponding
invflag. If zero, either negating or the option itself is not supported.
(In practice, a lookup for unsupported option won't happen as those are
caught by getopt_long()).

Since xtables-arp's OPT_* values change, adjust the local
inverse_for_options array accordingly.

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