Cygwin InstallXLogFileSegment() rename() patch

The attached patch fixes a problem with InstallXLogFileSegment()'s use
of link() under Cygwin:

    http://archives.postgresql.org/pgsql-cygwin/2002-04/msg00072.php

Note that it appears that BeOS and Netware also have the above or
similar problem.

I have only verified that PostgreSQL builds under Cygwin with this patch.
Since I cannot reproduce the problem, I cannot verify that the proposed
patch solves it.  Nevertheless, both Barry Pederson and David P. Caldwell
attest that this patch solves the problem.  See the following for details:

    http://archives.postgresql.org/pgsql-cygwin/2002-05/msg00043.php
    http://archives.postgresql.org/pgsql-cygwin/2002-05/msg00040.php

Thanks,
Jason