I'd like to see if we could get ap_runtime_dir_relative() into 2.4 in time for the release.
Right now, trunk includes that func and slotmem utilizes it, and trunk passes tests with DefaultRuntimeDir set to both an abs and relative value. If people can confirm that would be great... I can start the transition within the other modules that use DEFAULT_REL_RUNTIMEDIR to use the new api.
