RFR: 8028391 - Make the Min/MaxHeapFreeRatio flags manageable

[Jesper Wilhelmsson]

Hi,

Could I have a few reviews of this change?

Summary:
To allow applications a more fine grained control over the GC over time, we'll 
make the flags MinHeapFreeRatio and MaxHeapFreeRatio manageable.

The initial request that lead up to this change involved ParallelGC which is 
notoriously unwilling to shrink the heap. Since ParallelGC didn't support the 
heap free ratio flags, this change also includes implementing support for these 
flags in ParallelGC.

Changes have also been made to the argument parsing, attach listener and the 
management API to verify the flag values when set through the different interfaces.

Webrev: http://cr.openjdk.java.net/~jwilhelm/8028391/webrev.4/

Bug: https://bugs.openjdk.java.net/browse/JDK-8028391

The plan is to push this to 9 and then backport to 8 and 7.

Thanks!
/Jesper