Inline nd6_option() helper, remove indirections

Inline nd6_option() helper, remove indirections

Move the function body into the while loop, merge identical variables,
pull the `invalid' label out of the loop and straighten `skip' into the
`skip1' label.

Merging nd6_option() into nd6_options() is now much clearer after the
previous clean up.

nd_opts_{search,last,done} are now clearly "private" to n6_options() and
can be cleaned up from struct nd_opts next.

OK claudio