support/htcacheclean: Teach it how to write a pid file (modelled on

httpd's writing of a pid file) so that it becomes possible to run
more than one instance of htcacheclean on the same machine.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@910705 13f79535-47bb-0310-9956-ffa450edef68