MFC r354462,354643,354680,354731,354748-354750,354757,354831-354832,

MFC r354462,354643,354680,354731,354748-354750,354757,354831-354832,
    354861-354863,354865,355254,355450,355452

  netinet*: variable cleanup
  netinet*: update *mp to pass the proper value back
  nd6 defrouter: consolidate nd_defrouter manipulations in nd6_rtr.c
    (excluding some changes to keep public KPI)
  nd6: simplify code
  netinet6: Remove PULLDOWN_TESTs.
  netinet*: replace IP6_EXTHDR_GET()
  IP6_EXTHDR_CHECK(): remove the last instances
  nd6_rtr: consollidation and more consistent naming
  ipv6 tests: Add a simple ping6 test as well.
  icmpv6: Fix mbuf change in mld
  nd6_rtr: re-sort functions
  nd6: make nd6_timer_ch static
  nd6: sysctl
  in6: move include
  Fix m_pullup() problem after removing PULLDOWN_TESTs and KAME EXT* macros.
    (IP6_EXTHDR* macros are preserved in stable).
  ip6_input: remove redundant v4mapped check
  Update comment.

PR:		240135