RFR 9: 8176272 : (process) ProcessHandle::onExit fails to wait for non-child process

[Roger Riggs]

Please review a change to the implementation of ProcessHandle.onExit to
correctly handle waiting for any process to terminate and a 
corresponding new test.
On Linux, waitpid only waits for direct children; waiting for non-children
now polls for termination.
Please review and comment:

Webrev:
http://cr.openjdk.java.net/~rriggs/webrev-processhandle-onexit-8176272/index.html

Issue:
  https://bugs.openjdk.java.net/browse/JDK-8176272

Thanks, Roger