xtables: Derive xtables_globals from family

xtables: Derive xtables_globals from family

Prepare xtables_main() for use with other families than IPV4 or IPV6
which both use the same xtables_globals object. Therefore introduce a
function to map from family value to xtables_globals object pointer.

In do_parse(), use xt_params pointer as well instead of direct
reference.

While being at it, Declare arptables_globals and ebtables_globals in
xtables_multi.h which seems to be the proper place for that.

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