Don't fclose() the output in the backend

Don't fclose() the output in the backend

We fopen() the output file in common code but fclose() it in the
backend.  This is bad for a variety of reasons:

1. it is generally an awkward interface to change ownership.
2. we should use ferror() to test for write errors, and that is
   better done in common code.
3. it requires more code.
4. we still need to fclose() in common code during error handing.

Thus, move the fclose() of the output out of the backends, and add
fflush() so we can test ferror() on output.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>