Enable mod_mem_cache to be configured to cache open file handle using new

directive option: CacheEnable fd / will enable caching fds. If for some
reason the fd cannot be cached, the directive will effectively morph into
CacheEnable mem /


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94873 13f79535-47bb-0310-9956-ffa450edef68