Update CHANGES file to include the prefork idle child cleanup fix.

Changes with Apache 2.0.17-dev

  *) Changes required to make prefork clean up idle children properly.

     There was a window during which a starting worker deadlocks when

     an idle cleanup arrives before it completes init. Apache then keeps

     trying to cleanup the same deadlocked worker forever (until higher

     pids come along, but it still will never reduce below the deadlocked

     pid). Thus the number of children would not reduce to the correct

     idle level. [Paul J. Reder]

Changes with Apache 2.0.16

  *) Change the default installation directory to /usr/local/apache2,