More robust rrdtool_check_rrd_exists() check
Line return after outputting the rrd command for easier to read debug output
Updated some phpdoc comments in rrdtool.inc.php
Use data_update instead of rrd_update/rrd_create and influx_update
Centralize rrd file check so we can check against a remote rrdcached server too
Use rrd_name() to generate rrd file names
