Collectd meta graphs rrdcommand generated incorrectly

The functions for collectd meta graphs put rrdtool base command and subcommand in the command string. As these are handled outside for a while, the resulting command strings are not working and returning errors.

Also, in collectd_draw_meta_line the rrd_opts given via opts argument is not merged with the other options as it is done in collectd_draw_meta_stack.

The attached patches fix both of these issues separately.