inc/collectd: consistently return false instead of an empty array or false

This means we have to check the return value of these functions for
example before doing a foreach. We can't just do a foreach and assume the
return value of a function is an array, while it can be false and results
in a PHP warning.

Closes: https://github.com/pommi/CGP/pull/68
inc: mv graphs_from_plugin, build_url functions to html.inc.php

They generate html.