RFR JDK-8066709 Make some JDK system properties read only

[Roger Riggs]

Please review a change to make the values of java.home, user.home, 
user.dir, and user.name
effectively read-only for internal use.  The values are cached during 
initialization and the
cached values are used.

Webrev:
   http://cr.openjdk.java.net/~rriggs/webrev-static-property-8066709/

Issue:
   https://bugs.openjdk.java.net/browse/JDK-8066709

CSR:
   https://bugs.openjdk.java.net/browse/JDK-8204235

Thanks, Roger