mod_slotmem_plain: make the "filename" key relative to

DefaultRuntimeDir if not absolute

mod_slotmem_plain does not externalize or check the full path
in any way, so this change is just for consistency with other
modules which build paths to run-time artifacts.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1370763 13f79535-47bb-0310-9956-ffa450edef68