dhcp: delay restarts after NAKs

dhcp: delay restarts after NAKs

The server might answer to a DHCPREQUEST with a NAK and currently the
client restarts the configuration process immediately.  It was
observed that this can easily generate loops in which the network is
flooded with DISCOVER,OFFER,REQUEST,NAK sequences.

RFC 2131 only states that "if the client receives a DHCPNAK message,
the client restarts the configuration process" without further
details.

Add a delay with exponential backoff between retries after NAKs to
limit the number of requests and cap the delay to 30 minutes.