davautocheck.sh: teach it to re-use the same port every time.  I really
shouldn't have to copy the port number from the "run httpd under gdb" shell
to the "exercise it" shell by hand.

Note: this also makes davautocheck.sh auto-stop the last instance it started.

* subversion/tests/cmdline/davautocheck.sh
  (STOPSCRIPT): New filename.
  (random_port): Remove as unused.
  (HTTPD_PORT): Use the first available port ≥3691.
  (START): Before starting httpd, automatically stop the previous httpd
   launched by this script.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507909 13f79535-47bb-0310-9956-ffa450edef68