Shifts left and right by 32 were tripping a gcc warning (fatal with

Shifts left and right by 32 were tripping a gcc warning (fatal with
-Werror, of course) about shifting by more than the range of a data
type. They only appeared in `if' statements testing sizeof(off_t),
but gcc warns even when the code is unreachable. I've removed the
conditional code (the general case should still work even on 32-bit
machines), and hacked each shift by 32 into a pair of shifts by 16.

Note that the gcc warning is not just a helpful indication that you
may be using the wrong data type; it's actually pointing out ANSI-
undefined behaviour in shifting a signed integer beyond the size of
its type.

[originally from svn r6789]