support for v6 on the LAN

Note that udhcpc6 is not used because:
 * does not work with PPP interfaces, as the source address is set to '::'
  * dhcp6s sees these packets, but is unable to respond; sendmsg() works but packet vanishes
  * https://bugs.busybox.net/show_bug.cgi?id=6194
  * http://lists.busybox.net/pipermail/busybox/2013-August/079714.html
  * http://lists.busybox.net/pipermail/busybox/attachments/20130820/5a657f15/attachment.bin
 * no support for prefix delegation
 * no support for support RDNSS
 * d6_dhcpc.c:d6_find_option()/option_to_env() are bust and are unable to read options at the tail end
  * thinks it is a bogus packet
  * loop probably should look more like (which works)
      while (option < option_end) { if (option[3] > option_end - option - 4) return NULL; ...; option += option[3] + 4; }

Only down side is the odhcp6c binary is 60kB (mipsel) and we add UCLIBC_HAS_GLIBC_CUSTOM_STREAMS.
move /etc/nftables to /opt/bratwurst