RFR: 8022854 : (s) Cleaner re-initialization of System properties

[Mike Duigou]

Hello all;

This is a bit of cleanup I did back during Java 8 that got deferred due to it's late arrival during the development cycle. I've updated it for Java 9.

http://cr.openjdk.java.net/~mduigou/JDK-8022854/0/webrev/

This change improves the implementation of System.setProperties(null) which restores the system properties to their boot time defaults. The existing semantics are preserved.

Mike