i_divmod():  As discussed on Python-Dev, changed the overflow
checking to live happily with recent gcc optimizations that
assume signed integer arithmetic never overflows.

This differs from the corresponding change on the 2.5 and 2.4
branches, using a less obscure approach, but one that /may/
tickle platform idiocies in their definitions of LONG_MIN.
The 2.4 + 2.5 change avoided introducing a dependence on
LONG_MIN, at the cost of substantially goofier code.