RFR (S) 8217378: UseCriticalCMSThreadPriority is broken

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8217378

Fix:
  http://cr.openjdk.java.net/~shade/8217378/webrev.01/

While this concerns the experimental CMS option, the fix itself also unblocks Shenandoah RFE
(JDK-8217343). There might be a larger discussion if CriticalPriority thingie is useful, but I would
first push the simple fix that makes it work, at least to have the up-to-date performance data. It
should make the most benefit on Solaris where critical prio can map to FX priorities, but
non-Solaris builds can also enjoy the (control) GC threads priority elevated to MaxPriority, above
the VMThread.

Testing: new test, hotspot tier1, jdk-submit (running)

Thanks,
-Aleksey