[dhcp] Handle missing ProxyDHCP servers

It is possible to configure a DHCP server to hand out PXE options
without a ProxyDHCP server present.  This requires setting option 60
to "PXEClient", which will cause gPXE to attempt ProxyDHCP.

We assume in several places that dhcp->proxydhcpack is set to the
DHCPACK packet containing option 60 set to "PXEClient".  When we
transition into ProxyDHCPREQUEST, set dhcp->proxydhcpack=dhcp->dhcpack
so that this assumption holds true.

We ought to rename several references to "proxydhcp" to something more
accurate, such as "pxedhcp".  Treating a single DHCP response as
potentially both DHCPOFFER and ProxyDHCPOFFER does make the code
smaller, but the variable names get confusing.

src/net/udp/dhcp.c

 		break;
 	case DHCP_STATE_REQUEST:
 		if ( dhcp->proxydhcpoffer ) {
+			/* Store DHCPACK as ProxyDHCPACK.  This
+			 * handles the case in which the DHCP server
+			 * itself responds with "PXEClient" and PXE
+			 * options but there is no actual ProxyDHCP
+			 * server resident on the machine.
+			 */
+			dhcp->proxydhcpack = dhcpset_get ( dhcp->dhcpack );
 			dhcp_set_state ( dhcp, DHCP_STATE_PROXYREQUEST );
 			break;
 		}
 	proxydhcpack->settings.name = PROXYDHCP_SETTINGS_NAME;
 
 	/* Record ProxyDHCPACK */
-	assert ( dhcp->proxydhcpack == NULL );
+	dhcpset_put ( dhcp->proxydhcpack );
 	dhcp->proxydhcpack = dhcpset_get ( proxydhcpack );
 
 	/* Register settings */