[PATCH] Use access() to check file existence in GetNewRelFileNode().

Previous code uses BasicOpenFile() + close().

access() should be faster than BasicOpenFile()+close() and access()
should be more correct since BasicOpenFile() could fail for various
cases (e.g. due to file permission, etc) even the file exists.

access() is supported on Linux/Unix. I do not have a Windows dev
environment, but MSDN tells me that access() is supported on Windows also
and there have been access() call in the workspace, so I assume there is no
portability issue.

Thanks.