Allow MemManagerFile to use ServerRoot relative paths

MemManagerFile directive requires absolute path to directory where runtime data is stored.
The patch allows to have relative paths against ServerRoot.

One can use 'MemManagerFile cache/mod_proxy_cluster' instead 'MemManagerFile /var/cache/httpd/mod_proxy_cluster'. The patch allows current configurations that have absolute path to work as before, while ensuring that relative paths are created inside ServerRoot.