These changes seem to fix the linux32 build (for real)

Sorry for shooting in the blind on my previous commit -- I couldn't
figure out why my linux32 builds weren't matching the smoke-test ones;
turns out it was because I didn't have CHPL_DEVELOPER set on that
linux32 box (D'oh!).

Rather than pushing off_t through the callchains as started by my
previous approach, here I back off, realizing that doing so would
require changing HDFS, cURL, and Lustre interfaces (scary!)
Meanwhile, keeping the file length an int64_t only required changing
one other variable to fix the linux32 builds.  Whew!