apache_status: replace old Apache plugin with new, multigraph-capable one.

The new apache_status plugin provides the previous three graphs as
multigraphs, with a single request to Apache.

It also works as a virtual-node plugin, by querying a remote server.
snmp__df.in: convert to use Munin::Plugin::SNMP.

This basically removes lots of boilerplate code that is already
available in that module, and simplifies the plugin a lot. It does not
change the output the least.