Don't access the _offset member of FILE directly. Instead, use
ftell() to obtain the current file offset.
Note: I did not use ftello() since the rest of this file uses
fseek() and ftell(). Also, it might be more direct to use fstat()
to obtain the file's size rather than fseek() / ftell(), but I went
with the simpler change.
The goal here is to permit us to make FILE more opaque to minimize
future ABI compat shims.