xshared: Store optstring in xtables_globals

xshared: Store optstring in xtables_globals

Preparing for a common option parser, store the string of options for
each family inside the respective xtables_globals object. The
array of long option definitions sitting in there already indicates it's
the right place.

While being at it, drop '-m' support from arptables-nft.

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