RFR: JDK-8114823 - G1 doesn't honor request to disable class unloading

[Jesper Wilhelmsson]

Hi,

Please review this change to make the GCs disable class unloading if disabled on 
the command line.

There are a few parts in this change:

1. The flag ClassUnloadingWithConcurrentMark was not set to false if 
ClassUnloading was disabled on the command line. This is now done.

2. CMS was using CMSClassUnloadingEnabled instead of the more generic 
ClassUnloadingWithConcurrentMark. I changed so that 
ClassUnloadingWithConcurrentMark is used all over and CMSClassUnloadingEnabled 
is only used as an alias when initializing the arguments. At some point it would 
be nice to deprecate CMSClassUnloadingEnabled but it is a fairly well known flag 
so that's not part of this change.

3. CMS, Serial, and PS did correctly disable class unloading when told so, but 
the outermost code witch included some logging was still enabled making class 
unloading output present in the log file. I changed so that this code is also 
disabled.

4. G1 did not disable class unloading during full GCs at all. This is now done.

Testing: I used SecureDBBTest.java as suggested in the bug while fixing to 
verify that class unloading was happening before the fix and not after. I also 
ran it through JPRT.

Bug: https://bugs.openjdk.java.net/browse/JDK-8114823
Webrev: http://cr.openjdk.java.net/~jwilhelm/8114823/webrev.00/

Thanks,
/Jesper