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.

src/dhcp.c

@@ -3207,8 +3207,8 @@ dhcp_handledhcp(struct interface *ifp, struct bootp *bootp, size_t bootp_len,
 
 	if (has_option_mask(ifo->requestmask, DHO_IPV6_PREFERRED_ONLY)) {
 		if (get_option_uint32(ifp->ctx, &v6only_time, bootp, bootp_len,
-		    DHO_IPV6_PREFERRED_ONLY) == 0 &&
-		    (state->state == DHS_DISCOVER || state->state == DHS_REBOOT))
+		    DHO_IPV6_PREFERRED_ONLY) == 0 && (state->state == DHS_DISCOVER ||
+		    state->state == DHS_REBOOT || state->state == DHS_NONE))
 		{
 			char v6msg[128];