busybox: Parse multiple IPv6 addresses in dhcp6

This patches the udhcpc6 application to add environment variables for
each of the IA_ADDR blocks in the DHCP adverisement instead of just the
first one. It keeps the same variable for the 0th address to retain
backwards compatability. You can expect the mapping
  IA_ADDR[0] -> $ipv6_0
  IA_ADDR[1] -> $ipv6_1
  ...
  IA_ADDR[n] -> $ipv6_n

Tested: Verified working with multi-IP DHCP by manually triggering
udhcp6 with environment info printed during an installation job.
Fusion-Link: fusion2 N/A

Google-Bug-Id: 428779896
Change-Id: If4f3a5584eecf8b5f822a92144a01e4996963452
Signed-off-by: William A. Kennington III <wak@google.com>