plugins: introduce a readfile() function to Munin::Plugin

Especially on Linux where most of the information for the system
status data is read out of /sys or /proc, it's easiest to have a
convenience function that reads a whole file in a scalar variable.
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).
uptime: rewrite plugin in Perl

The new version no longer has to fork and use awk to process
/proc/uptime, but handles everything within Perl. Furthermore it
supports dirtyconfig.
cpu: rewrite the plugin in Perl.

The new plugin is implemented purely in Perl, so it avoids the three
egrep calls per execution, the awk and so on. It's also much more
easily extended (and indeed supports the new guest_nice available on
3.7 at least).

The new plugin is also shorter than before.