Oops, this should have been committed to head first and the 2.4 branch

Oops, this should have been committed to head first and the 2.4 branch
second.

I am having some infrequent coredumps on DEC Alphas.  Somehow, shared
memory pointers are gaining a high bit set, almost as though due to
an addition/subtraction overflow.  One potential problem is that I
was storing the value of long pointer subtraction into a 32-bit integer.
I changed those integer offsets to off_t.  However, the offset value
in the inter-process messages remains 32-bits to keep the message size
small and predictable.