So we want to stop SystemExit from causing
an actual exit() when using python -i.
This patch introduces two new API calls,
PyRun_BlockSysExit and PyRun_UnblockSysExit,
to set a flag toavoid the exit() call in
PyErr_PrintEx.

There are several other ways to fix this
bug, but I think all of the others I came up
with would cause more backwards compatibilty
problems and/or be a lot more work.
Some possibilities, if anyone is interested,
would be:
1) Add a new PyCompilerFlags flag
This seems a bit ugly, as it's not really
a "compile flag".
2) Add some special run routines that
block the exit.
3) Add another parameter to existing
run routines.
4) Change PyErr_PrintEx so it doesn't
exit when printing a SystemError, instead
having the run routines responsible for
exiting when catching that exception.

What do you think?  Is my patch good enough,
or would you like something else?

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While it works for me, I am not sure whether this is too
much overhead for fixing a glitch no one else complained about.

Committed a variant as rev. 54188.