sys/netinet6/nd6.c: add ND6_IFF_ACCEPT_RTADV flag

[Jeffrey Hsu]

Hiroki Sato wrote:
Hiroki Sato <hrs at xxxxxxxxxx> wrote
  in <20041229.071648.02292479.hrs at xxxxxxxxxxxxx>:
hrs>  Changes to the userland utility are needed, too.  KAME implements
hrs>  ndp(8) to handle the flag and I think I will submit the patch soon.
 The patch is attached.
Committed.  Thanks for your submission.

(I also updated ndp.8 while I was at it.)