RFR: 8250646: hdiutil detach fix JDK-8245311 still fails sometimes

[alexander.matveev at oracle.com]

Please review the jpackage fix for bug [1] at [2].

- DMG image can get detach even if "hdiutil detach" return resource busy 
error. Fixed by stopping trying detach if DMG already got detach.

[1] https://bugs.openjdk.java.net/browse/JDK-8250646
[2] http://cr.openjdk.java.net/~almatvee/8250646/webrev.00/

Thanks,
Alexander