Patch for ar in binutils-2.15.91-20040904-1

The ar command crashes when used with MRI scripts and
the ADDLIB command. This crash is specific to Windows,
as it tries to rename a file into another which is left
opened.

The fix provided is not Win32-specific, as it cleanly
closes the opened file before being removed or renamed
to (UNIX implicitely closes it on exit).

There is a ar.mri script attached showing the problem
under a standard MinGW installation. Notice that you
can run this test on Linux as well by changing the
ADDLIB library in the MRI script. The bug shows up by
typing 'ar -M <ar.mri'.

Thank you !