[Stackless] st.serial --> st.serial_last_jump patch

[Jeff Senn]

Kristjan (or anyone else if you've looked at it)-

RE: Your serial_last_jump patch...

In my attempt to go to 2.6.4 I included your patch -- but it causes a crash
(that goes away when I revert it out).  I haven't debugged much -- but what
appears to be happening is that serial_last_jump != serial in the main tasklet
cleanup causing an unnecessary slp_transfer_return... (in my case there is
only soft-switching and one thread that is known to python).

It does happen in a case that one would not normally find:  in particular
I have stackless embedded in an app that make an instance of the interpreter
and "feeds" it work to do... occasionally when it's "done", 
the main tasklet exits back to the controlling program -- but the interpreter 
can be called  again in the future -- creating a new main tasket...etc...

It is one one of these subsequent main tasklet exits that the unnecessary
slp_transfer_return causes a crash.

I'll continue to debug at some rate, but if you have some idea what might be
wrong, please let me know...