Honor DefaultRuntimeDir for mutexes, socache and CGID socket.

Move away from DEFAULT_REL_RUNTIMEDIR and use
ap_runtime_dir_relative() API.

Backport of r1299718 from trunk.

Submitted by: jim
Reviwed by: rjung, trawick
Backported by: rjung



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1364709 13f79535-47bb-0310-9956-ffa450edef68