RFR (S): 8243936: NonWriteable system properties are actually writeable

[David Holmes]

webrev: http://cr.openjdk.java.net/~dholmes/8243936/webrev/
bug: https://bugs.openjdk.java.net/browse/JDK-8243936

Please see details in the bug report.

tld;dr: we again call the set method that checks whether a flag is 
writeable, and I added the equivalent append version. Added some 
commentary about the methods used only for module options and changed 
the basic module properties to be writeable as they need to allow 
overriding.

Testing:
   - added a new test that verifies you can't set a non-writeable 
property. (There's no test for the append case because we don't have any 
appendable non-writeable flags)
  - ran all the module related tests I could find to ensure no impact on 
the module flags
   - jdk/tools/launcher/modules/
   - jdk/com/sun/tools/attach/modules
   - jdk/java/lang/Class/forName/modules
   - jdk/java/lang/ClassLoader/getResource/modules
   - jdk/java/lang/SecurityManager/modules
   - jdk/java/lang/System/LoggerFinder/modules
   - jdk/java/lang/instrument/modules
   - jdk/java/lang/invoke/modules
   - jdk/java/sql/modules
   - jdk/java/util/ResourceBundle/modules
   - jdk/java/util/ServiceLoader/modules
   - jdk/java/util/logging/modules
   - jdk/javax/security/auth/login/modules
   - jdk/jdk/jfr/api/modules
   - jdk/jdk/modules
   - hotspot/jtreg/runtime/modules/
  - tiers 1-3

Thanks,
David