Fix DHCP relay, broken by ff325644

Thanks to John Fitzgibbon for initial patch.

CHANGELOG

+version 2.78
+        Fix logic of appending ".<layer>" to PXE basename. Thanks
+	to Chris Novakovic for the patch.
+
+	Revert ping-check of address in DHCPDISCOVER if there
+	already exists a lease for the address. Under some
+	circumstances, and netbooted windows installation can reply
+	to pings before if has a DHCP lease and block allocation
+	of the address it already used during netboot. Thanks to
+	Jan Psota for spotting this.
+
+	Fix DHCP relaying, broken in 2.76 and 2.77 by commit
+	ff325644c7afae2588583f935f4ea9b9694eb52e. Thanks to
+	John Fitzgibbon for the diagnosis and patch.
+	
+	
 version 2.77
 	    Generate an error when configured with a CNAME loop,
 	    rather than a crash. Thanks to George Metz for

src/dhcp.c

@@ -273,8 +273,8 @@ void dhcp_packet(time_t now, int pxe_fd)
   if ((relay = relay_reply4((struct dhcp_packet *)daemon->dhcp_packet.iov_base, ifr.ifr_name)))
     {
       /* Reply from server, using us as relay. */
-      iface_index = relay->iface_index;
-      if (!indextoname(daemon->dhcpfd, iface_index, ifr.ifr_name))
+      rcvd_iface_index = relay->iface_index;
+      if (!indextoname(daemon->dhcpfd, rcvd_iface_index, ifr.ifr_name))
 	return;
       is_relay_reply = 1; 
       iov.iov_len = sz;