RRD base options are not included in collectd include

When viewing the collectd tab on a high resolution display the graphs are not rendered properly. They are lacking the the RRD option "--zoom".

A comparison of the file collectd.inc.php with the generic graph include files showed that the file collectd.inc.php lacks including the file common.inc.php. Also, it sets the variable "rrd_opts" instead of appending to it.

The attached patch adds the include and changes the handling of the variable "rrd_opts", so the generic base rrd options, like "--zoom", are present for collectd graphs as well.