- Refactoring the interface list. It now supports dynamically

- Refactoring the interface list.  It now supports dynamically
  added/removed interfaces in a more consistent manner and reloading
  the configuration file.

- Add initial support for control socket.  RA information in the
  daemon can be obtained by rtadvctl(8) instead of SIGUSR1 in a similar
  manner to ifconfig(8).  The information dump has been removed in favor of it.

  (reload the configuration file)
  # rtadvctl reload

  (show RA messages being sent on each interfaces)
  # rtadvctl show
  em0: flags=<UP,CONFIGURED,PERSIST> status=<RA_SEND> mtu 1280
        DefaultLifetime: 30m
        MinAdvInterval/MaxAdvInterval: 3m20s/3m20s
        AdvLinkMTU: <none>, Flags: O, Preference: medium
        ReachableTime: 0s, RetransTimer: 0s, CurHopLimit: 64
        AdvIfPrefixes: yes

  (show RA messages being sent only on em0)
  # rtadvctl show em0

  (rtadvctl -v show provides additional information)
  # rtadvctl -v show em0
  em0: flags=<UP,CONFIGURED,PERSIST> status=<RA_SEND> mtu 1280
        DefaultLifetime: 30m
        MinAdvInterval/MaxAdvInterval: 3m20s/3m20s
        AdvLinkMTU: <none>, Flags: O, Preference: medium
        ReachableTime: 0s, RetransTimer: 0s, CurHopLimit: 64
        AdvIfPrefixes: yes
        Prefixes (1):
          2001:db8:1::/64 (CONFIG, vltime=30d, pltime=7d, flags=LA)
        RDNSS entries:
          2001:db8:1::128 (ltime=2m40s)

  (stop rtadvd)
  # rtadvctl shutdown

  A remaining issue when reloading the configuration file is that
  during that period rtadvd cannot communicate with rtadvctl due to some
  additional RA sending for graceful shutdown.  This will be fixed later.