dhcp: Fix dhcp issue on VBOX

I've seen OSv can not get dhcp address in Virtual Box.

On Linux, the dhcp client sends dhcp options in both DISCOVER and
REQUEST packets. However, OSv only sends the options in DISCOVER
packet.

Fix the issue by making OSv behave the same with Linux dhcp client.

Though other people (Vlad) did not this issue, it won't hurt to send
options in both packets.

This patch fixes the dhcp issue for me.

Reviewed-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>