[dhcp] Include session state metadata in packet traces

(Ab)use the "secs" field in transmitted DHCP packets to convey
metadata about the DHCP session state.  In particular:

  bit 0 represents the receipt of a ProxyDHCPOFFER
  bit 1 represents the receipt of a DHCPOFFER
  bits 2+ represent the transmitted packet sequence number

This allows some relevant information about the internal state of the
DHCP session to be read out from a packet trace from a non-debug build
of iPXE.  It also potentially allows replies to be correlated to their
requests (for servers that copy the "secs" field from request to
reply).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp/dhcp.c

@@ -245,6 +245,8 @@ struct dhcp_session {
 
 	/** Retransmission timer */
 	struct retry_timer timer;
+	/** Transmission counter */
+	unsigned int count;
 	/** Start time of the current state (in ticks) */
 	unsigned long start;
 };
@@ -1098,6 +1100,14 @@ static int dhcp_tx ( struct dhcp_session *dhcp ) {
 		goto done;
 	}
 
+	/* (Ab)use the "secs" field to convey metadata about the DHCP
+	 * session state into packet traces.  Useful for extracting
+	 * debug information from non-debug builds.
+	 */
+	dhcppkt.dhcphdr->secs = htons ( ( ++(dhcp->count) << 2 ) |
+					( dhcp->offer.s_addr ? 0x02 : 0 ) |
+					( dhcp->proxy_offer ? 0x01 : 0 ) );
+
 	/* Fill in packet based on current state */
 	if ( ( rc = dhcp->state->tx ( dhcp, &dhcppkt, &peer ) ) != 0 ) {
 		DBGC ( dhcp, "DHCP %p could not fill DHCP request: %s\n",