Simplify nd6_options() initialise logic

Simplify nd6_options() initialise logic

nd_opts_{search,last,done} are exlusively used in the internal option
handling machinery;  the only two nd6_options() callers only use
nd_opts_{src,tgt}_lladdr.

nd6_options() always zeroes and initialises the caller's struct nd_opts.
If icmp6len is zero, i.e. if there are no ICMP6 header options left,
everything inside *ndopts is zero, except nd_opts_done=1 which is not
used by the callers.

Set the internal nd_opts_{search,last,done} members only when needed.

OK claudio