asterisk: introduce a new, multigraph-capable plugin for asterisk.

The new plugin replaces all seven of the old plugins, with a single,
common and collapsed configuration, by drawing multiple graphs with a
single set of transactions to the server.

It also supports autoconf (which was not supported by the old asterisk
plugins), and wildcard/virtual node mode, when monitoring a remote
server.
apache: 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: 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.
squid: introduce new Squid plugin to replace the old ones.

Again this plugin uses multigraph and supports virtualnode, and
collapses the five previous squid plugins, reducing the number of
requests coming in for the proxy itself.

Neither output nor configuration ports directly to the new plugin,
unfortunately, but it does allow for saner output and store (in
particular killing cdefs).
entropy: rewrite plugin in Perl.

This allows us to not have to execute multiple commands to be able to
get the entropy (which could, by themselves, deplete it!).

Additionally, this version fixes the vlabel, and sets the upper limit,
as well as the maximum reading values, to the poolsize.

Since the poolsize is constant for a given kernel, this allows to have
a better, proportioned view of how the entropy varies in the system.
hwmon: introduce a new, multigraph and /sys-based plugin for hardware sensors.

This plugin replaces both the acpi thermal zone plugin and the
lm_sensors based one. It will graph the same data, even though with
different labels (but that involves mostly legacy systems).