[dhcp] Handle DHCPNAK by returning to discovery state

[dhcp] Handle DHCPNAK by returning to discovery state

Handle a DHCPNAK by returning to the discovery state to allow iPXE to
attempt to obtain a replacement IPv4 address.

Reuse the existing logic for deferring discovery when the link is
blocked: this avoids hammering a misconfigured DHCP server with a
non-stop stream of requests and allows the DHCP process to eventually
time out and fail.

Originally-implemented-by: Blake Rouse <blake.rouse@canonical.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>