Merge r1595549:

    Some improvements to the pool concurrency check code

    * use defined constants for in_use
    * don't use C99's __func__
    * improve abort message
    * fix some confusion wrt the destroyed state



git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.6.x@1595550 13f79535-47bb-0310-9956-ffa450edef68