Fix #12530: detect Windows if closing RFS.

This change ultimately short-circuits the ACTIVE_CLOSE setting, by not requiring the user to set it explicitly via JAVA_OPTS. We are aware of the RawFileStore closing requirement on Windows, hence we do it when running on Windows.

Test:
- in-place import (ln, ln_s and ln_rm) on Windows and *nix systems. I've verified that the code behaves as per our documentation on OS X and Windows 7.