update IVT when relocating inthandlers

[stsp]

When relocating the int handler, we need to update
IVT entry so that it always points to the handler, not
to PLT. This is needed so that the shared headers
can be added.
But first, it must be checked if its not the case already.

[stsp]

The idea of updating IVT didn't work: freedos
relocates to HMA after config.sys is processed
(to get the use of HMA driver, like ems.sys in
our case), which is too late. All loaded drivers have
remembered the int handler entries at that point.
So instead I added the shared headers right to
the relocation stubs.
You can now track the handlers (see int21/int2f
for instance) up to their HMA locations.