MFC r266800 by vanhu:
  IPv4-in-IPv6 and IPv6-in-IPv4 IPsec tunnels.
  For IPv6-in-IPv4, you may need to do the following command
  on the tunnel interface if it is configured as IPv4 only:
  ifconfig <interface> inet6 -ifdisabled

  Code logic inspired from NetBSD.
  PR: kern/169438

MC r266822 by bz:
  Use IPv4 statistics in ipsec4_process_packet() rather than the IPv6
  version.  This also unbreaks the NOINET6 builds after r266800.

MFC r268083 by zec:
  The assumption in ipsec4_process_packet() that the payload may be
  only IPv4 is wrong, so check the IP version before mangling the
  payload header.

MFC r272394:
  Do not strip outer header when operating in transport mode.
  Instead requeue mbuf back to IPv4 protocol handler. If there is one extra IP-IP
  encapsulation, it will be handled with tunneling interface. And thus proper
  interface will be exposed into mbuf's rcvif. Also, tcpdump that listens on tunneling
  interface will see packets in both directions.

PR:		194761