Change Windows file.truncate() to (a) restore the original file position,
and (b) stop trying to prevent file growth.

Beef up the file.truncate() docs.

Change test_largefile.py to stop assuming that f.truncate() moves the
file pointer to the truncation point, and to verify instead that it leaves
the file position alone.  Remove the test for what happens when a
specified size exceeds the original file size (it's ill-defined, according
to the Single Unix Spec).
3 files changed