net: dhcpv4_server: Add option to NAK Request from unrecognized client

Zephyr's DHCPv4 server does not implement persistent storage of address
leases. In result, all leases are lost on reboot, which can cause
delays with clients starting in INIT-REBOOT state and thus sending
(potentially several) Requests before attempting full Discover-Request
procedure.
Add option to override RFC defined behavior, which states that if we
don't recognize the client sending the Request, the server shall remain
silent. Enabling that option allows the server to send NAK reply in case
client is not recognized, informing the client it should proceed with
full procedure.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>