MFC r222732,222771,222820,222824,222972,223752,224006,224144,224148,225519,22568

MFC r222732,222771,222820,222824,222972,223752,224006,224144,224148,225519,22568
3,228990:

- Implement RDNSS and DNSSL options (RFC 6106, IPv6 Router Advertisement
  Options for DNS Configuration).

- rtadvd(8) now supports "noifprefix" to disable gathering on-link prefixes
  from interfaces when no "addr" is specified[2].  An entry in rtadvd.conf
  with "noifprefix" + no "addr" generates an RA message with no prefix
  information option.

- rtadvd(8) now supports RTM_IFANNOUNCE message to fix crashes when an
  interface is added or removed.

- Implement burst unsolicited RA sending into the internal RA timer framework
  when AdvSendAdvertisements and/or configuration entries are changed as
  described in RFC 4861 6.2.4.  This fixes issues that make termination of the
  rtadvd(8) daemon take very long time.

- rtadvd(8) now accepts non-existent interfaces as well in the command line.

- Add control socket support and rtadvctl(8) utility to show the RA information
  in rtadvd(8).  Dumping by SIGUSR1 has been removed in favor of it.

git-svn-id: svn://svn.freebsd.org/base/stable/8@231802 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f