[syslinux] Bugs in pxelinux.asm - syslinux 3.75

[pfranz73 at tiscali.it]

In pxelinux.asm
the xchg instruction in

                xchg ax,ax
.
data_on_top:

should be xchg ax,dx, I think
At the end of 
pxe_get_cached_info routine,there is
                and ax,ax

                jnz .err
It is supposed to test for AX status, but 
since pxenv does pushad and popad, AX doesn't contain status. In fact 
the other routines calling pxenv don't have this bug.
pxe_unload seems 
to be never called; if so, also RealBaseMem is unused.
According to the 
specification, pxe_unload_stack_pkt.reserved are 10 bytes, not 10 words