RFR: JDK-8215550: Debugger does not work after reattach

[Gary Adams]

Significant protections are put in place to protect the commandLoop
from multiple events that that have a suspend-all policy. The
commandLoop uses a special block variable to ensure only
a VirtualMachine or ThreadReference call to resume() will unblock
the commandLoop. See

   src/jdk.jdwp.agent/share/native/libjdwp/eventHelper.c

In this particular bug report, the user was stopped at a breakpoint
when they sent the "exit" command. The same effect can be produced
with a "quit" command or any killing of the debugger process.

When the second session is started the commandLoop is still blocked,
so a new breakpoint will never be dequeued from the commandQueue.

The proposed workaround will ensure the commandLoop is unblocked
when a new session is started.

   Issue: https://bugs.openjdk.java.net/browse/JDK-8215550
   Webrev: http://cr.openjdk.java.net/~gadams/8215550/webrev.00/

All testing has been done by manually replicating the reported
command sequences. I'll see if an existing breakpoint test can be
enhanced to cover this scenario.