RFR(S): 8068579: Running with -XX:-UseParallelGC does not turn ParalleGC off

[Per Liden]

Summary: Explicitly disabling the default collector, without specifying 
another collector, will silently re-enable the default collector. This 
is a bit counter intuitive. A better approach would be for the VM to 
complain about the situation.

Bug: https://bugs.openjdk.java.net/browse/JDK-8068579
Webrev: http://cr.openjdk.java.net/~pliden/8068579/webrev.0/
Testing: manual, jprt

Example (assuming it's a platform/configuration with G1 as default):

$ java -XX:-UseG1GC ...

Today, the above command will happily start a VM with G1 enabled.

With this patch the VM will instead complain, like this:

$ java -XX:-UseG1GC ...
Error occurred during initialization of VM
Garbage collector not selected (default collector explicitly disabled)

And of course, we're good with disabling the default collector as long 
as you specify some other collector, e.g.:

$ java -XX:-UseG1GC -XX:+UseParallelGC ...

cheers,
Per