add feature to show a subset of graphs from a plugin on overview page
conf: disable overview_filter by default

It's good to have an example, but it's a bit confusing having it enabled by
default.
rewrite some code of added overview_filter feature

Use less variables, less code.
Merge remote-tracking branch 'pommi/master' into next

Resolved Conflicts:
	conf/config.php
	host.php
	inc/collectd.inc.php