RFR (S) 8226608: Hide the onjcmd option from the help output

[Schmelter, Ralf]

This change hides the onjcmd option of the JDWP agent and ensures that the corresponding VM.start_java_debugging jcmd is not listed via the help command. Both are still working though.

webrev: http://cr.openjdk.java.net/~rschmelter/webrevs/8226608/webrev.0/
bugreport: https://bugs.openjdk.java.net/browse/JDK-8226608

Best regards,
Ralf
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