Clean up struct nd_opts, use nd6_options() function local variables

Clean up struct nd_opts, use nd6_options() function local variables

nd_opts_search is really the next option, so call it next_opt.

nd_opts_done == 1 means next_opt == NULL, i.e. no more option to handle,
so zap the former and use the latter to stop.

Finally drop the useless struct members, all under _KERNEL.

OK claudio