Backport:

core: Add ap_runtime_dir_relative() and DefaultRuntimeDir.
Submitted by: trawick
Reviewed by: jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1305164 13f79535-47bb-0310-9956-ffa450edef68