fixups for remote RRDCACHED_ADDRESS environment variable and absolute vs. relative path flush issues

Hi.  This is a simple patch for fixing the case when the RRDCACHED_ADDRESS environment variable is set in the webserver environment (eg: export in init script or SetEnv in apache confs) to point at a remote rrdcached daemon.  In that case, the paths need to be fixed up to be relative to the datadir(s), otherwise the calls to RRDs::graph() fail with an error.

Let me know if you have any questions.

Thanks,
Brian