Respect IPV6_PREFERRED_ONLY flag on DHS_NONE state

Respect IPV6_PREFERRED_ONLY flag on DHS_NONE state

Current IPv6_PREFERRED_ONLY (option 108) handling code is only effective
when current state is DHS_DISCOVER and DHS_REBOOT. However, when we
receive multiple ACKs upon our REQUEST, the first ACK will trigger the
use_v6only code path and dhcp_drop() us into DHS_NONE state, as a result
the option 108 on the second ACK won't be handled correctly and we'll
bind to the lease instead.

This patch fixes the issue by adding DHS_NONE as a state to respect
option 108 as well.