MFC r266494:

  - Fix a bug which can make sysctl() fail when -F is specified.
  - Increase WID_IF_DEFAULT() from 6 to 8 (the default for AF_INET6) because
    we have interfaces with longer names than 6 chars like epairN{a,b}.
  - Style fixes.
MFC r266857:

  - Add rwlock to struct dadq.  A panic could occur when a large number of
    addresses performed DAD at the same time.