[syslinux] gpxelinux.0: pxechain to another NBP then RET fails

[Gene Cumm]

IRC user ernini first noticed this.  We both tried using
gpxe/gpxelinux.0 from Syslinux-4.04-pre22.  ernini used pxechain to a
commercial NBP which RETs (the same as PXELINUX) and failed.  Both of
us had success with core/pxelinux.0 from Syslinux-4.04-pre22.

For my test, I have a VM in VMware Server 2.0.2 (VMHW v7).  I loaded
gpxelinux.0 using the VM's built-in PXE ROM.  Then I used pxechain.com
to load PXELINUX 4.03 from another directory on the same TFTP server.
At this point, I tried LOCALBOOT -1 and 0.  -1 (which executes INT
19h) succeeded while 0 failed.  Ctrl-Alt-Del does not reboot the
machine.

I then used gpxe/pxelinux.gpxe and core/pxelinux.0 from
Syslinux-4.04-pre22 and gPXE to test different gPXE versions.  Using
1.0.1 results in empty packets being returned to PXELINUX.  Using
current top-of-git (commit c780ec8) resolves both issues.

-- 
-Gene