[dhcp] Do not restrict minimum retry time for ProxyDHCPREQUEST

The ProxyDHCPREQUEST is a unicast packet, so the first request will
almost always be lost due to not having the IP address in the ARP
cache.  If the minimum retry time is set to one second (as per commit
ff2b6a5), then ProxyDHCP will time out and give up before managing to
successfully transmit a request.

The DHCP timers need to be reworked anyway, so this mild hack is
acceptable for now.

src/net/udp/dhcp.c

@@ -818,6 +818,7 @@ static void dhcp_rx_dhcpack ( struct dhcp_session *dhcp,
 
 	/* If we have a ProxyDHCPOFFER, transition to PROXYDHCPREQUEST */
 	if ( dhcp->proxydhcpoffer ) {
+		dhcp->timer.min_timeout = 0;
 		dhcp_set_state ( dhcp, DHCP_STATE_PROXYREQUEST );
 		return;
 	}