[DHCP] Add support for ProxyDHCP requests

The PXE spec is (as usual) unclear on precisely when ProxyDHCPREQUESTs
should be issued.  We adapt the following, slightly paranoid approach:

  If an offer contains an IP address, then it is a normal DHCPOFFER.

  If an offer contains an option #60 "PXEClient", then it is a
  ProxyDHCPOFFER.  Note that the same packet can be both a normal
  DHCPOFFER and a ProxyDHCPOFFER.

  After receiving the normal DHCPACK, if we have received a
  ProxyDHCPOFFER, we unicast a ProxyDHCPREQUEST back to the ProxyDHCP
  server on port 4011.  If we time out waiting for a ProxyDHCPACK, we
  treat this as a non-fatal error.

src/include/gpxe/dhcp.h

 /** BOOTP/DHCP client port */
 #define BOOTPC_PORT 68
 
+/** ProxyDHCP server port */
+#define PROXYDHCP_PORT 4011
+
 /** Construct a tag value for an encapsulated option
  *
  * This tag value can be passed to Etherboot functions when searching
  */
 #define DHCP_MIN_LEN 552
 
-/** Maximum time that we will wait for ProxyDHCP offers */
+/** Maximum time that we will wait for ProxyDHCP responses */
 #define PROXYDHCP_WAIT_TIME ( TICKS_PER_SEC * 1 )
 
 /** Settings block name used for DHCP responses */
 /** Settings block name used for ProxyDHCP responses */
 #define PROXYDHCP_SETTINGS_NAME "proxydhcp"
 
-extern int create_dhcp_packet ( struct dhcp_packet *dhcppkt,
+extern int dhcp_create_packet ( struct dhcp_packet *dhcppkt,
 				struct net_device *netdev, uint8_t msgtype,
 				struct dhcp_options *options, 
 				void *data, size_t max_len );
-extern int create_dhcp_request ( struct dhcp_packet *dhcppkt,
+extern int dhcp_create_request ( struct dhcp_packet *dhcppkt,
 				 struct net_device *netdev,
+				 struct in_addr ciaddr,
 				 struct dhcp_packet *dhcpoffer,
 				 void *data, size_t max_len );
 extern int start_dhcp ( struct job_interface *job, struct net_device *netdev );

src/net/fakedhcp.c

 int create_fakedhcpdiscover ( struct net_device *netdev,
 			      void *data, size_t max_len ) {
 	struct dhcp_packet dhcppkt;
+	struct in_addr ciaddr = { 0 };
 	int rc;
 
-	if ( ( rc = create_dhcp_request ( &dhcppkt, netdev, NULL, data,
+	if ( ( rc = dhcp_create_request ( &dhcppkt, netdev, ciaddr, NULL, data,
 					  max_len ) ) != 0 ) {
 		DBG ( "Could not create DHCPDISCOVER: %s\n",
 		      strerror ( rc ) );
 	int rc;
 
 	/* Create base DHCPACK packet */
-	if ( ( rc = create_dhcp_packet ( &dhcppkt, netdev, DHCPACK, NULL,
+	if ( ( rc = dhcp_create_packet ( &dhcppkt, netdev, DHCPACK, NULL,
 					 data, max_len ) ) != 0 ) {
 		DBG ( "Could not create DHCPACK: %s\n", strerror ( rc ) );
 		return rc;
 	}
 
 	/* Create base DHCPACK packet */
-	if ( ( rc = create_dhcp_packet ( &dhcppkt, netdev, DHCPACK, NULL,
+	if ( ( rc = dhcp_create_packet ( &dhcppkt, netdev, DHCPACK, NULL,
 					 data, max_len ) ) != 0 ) {
 		DBG ( "Could not create ProxyDHCPACK: %s\n",
 		      strerror ( rc ) );