RFR 8138748: ManagementAgent.status DCMD fails with NPE for JMX configured on command line

[Jaroslav Bachorik]

Please, review the following change

Issue : https://bugs.openjdk.java.net/browse/JDK-8138748
Webrev: http://cr.openjdk.java.net/~jbachorik/8138748/webrev.00

The ManagementAgent.status DCMD, introduced in JDK 9, doesn't play 
nicely with the management agent being initialized via system 
properties. This patch adjusts the implementation to detect this kind of 
initialization and report the status appropriately.

The change has passed the local testing and a full JPRT run.

Thanks!

-JB-