Make JMX acceptor thread run as daemon thread.

Set the magic "run the acceptor thread as daemon" property that is
semi-documented in http://bugs.sun.com/view_bug.do?bug_id=6760712 and
http://www.docjar.com/src/api/com/sun/jmx/remote/util/EnvHelp.java.

This only works on JDK 7.