davautocheck.sh: reuse the same port number when stopping a previous instance.

* subversion/tests/cmdline/davautocheck.sh:
    Stop the previous instance before choosing an available port.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507946 13f79535-47bb-0310-9956-ffa450edef68