Split mod_disk_cache.c into mod_disk_cache.c and mod_disk_cache.h

      to allow inclusion of mod_disk_cache.h in htcacheclean.c.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@239406 13f79535-47bb-0310-9956-ffa450edef68