Import dhcpcd-6.4.0 with the following changes:

  *  Only fork when the RA has a RDNSS option AND a valid prefix OR no DHCPv6 instruction.
  *  Handle BSD per interface rtadv kernel flag.
  *  Add a reject route for the delegated prefix so that unassigned prefixes don't go back upstream.
  *  If not specified, Delegated Prefixes will get an automatic SLA of the interface index. If the biggest SLA and the assigned prefix fits into a /64 then dhcpcd creates a /64 prefix so that SLAAC works. If bigger than /64 is needed then dhcpcd creates one rounded upto the nearest multiple of 8. Unless a configured SLA of 0 is assigned, a reject route for the Delegated Prefix is installed to stop unassigned addresses trying to be resolved upstream. Addresses added from Delegated Prefixes now have a default address suffix of 1 instead of using a SLAAC style address.
  *  Don't spam timezone updates unless its actually changed.
  *  Support ND6_IFF_OVERRIDE_RTADV on BSD.
  *  Document why we don't send both FQDN and hostname options.
  *  Respect RFC4861 MAX_RTR_SOLICITATION_DELAY as specified in section 6.3.7.
  *  RFC2131 section 4.4.1 states the client SHOULD wait a random time between one and ten seconds to desynchronize the use of DHCP at startup. Instead we wait a random time between zero and one second to mirror the more modern IPv6RS and DHCPv6 standards unless overridden by defining RFC2131_STRICT.
  *  Always send a carrier as it's no longer encapsulated in the IPv4 stack. When stopping and not departed, run with the STOPPED reason but do not process it in hooks.
  *  Poll kernel neighbour reachability (SIOCGNBRINFO_IN6) for each router instead of sending and listening for Neighbour Soliciation/Advertisement packets. The kernel is privy to a lot more reachability information than userland is.
  *  Ensure that ND6_IFF_PERFORMNUD is set.
  *  controlgroup option changes group ownership of the control socket.
  *  Should use arc4random_uniform when wanting a randon number between 1 and N. Improve the compat arc4random function a little and re-stir on fork.
  *  -4 and -6 are now mutually exclusive and when running on a single interface per protocol pidfiles are created. This means that other control options suchs as -x and -n will require the -4 or -6 option as well.
  *  Implement Stable Private Addresses for SLAAC as per RFC7217. Enable this as default in dhcpcd.conf.
  *  Log error condition if a new or changed Router Advertisment to avoid spamming the log.
  *  Add an IPv6 link-local address before upping the interface if needed. Bring an interface up when we start it and not during discovery. As such, stop waiting on carrier for a second if we did.
  *  Add gateway option to enable the gateway or an interface/profile. Ensure the option and require options disable the nooption option - last wins.
  *  Restarting ntp/yp are not critical and their initialisation continues in the background as well so we may as well background the restart commands for faster dhcpcd performance.