Plugin.pm: introduce a readarray() function

To read one-liners for /sys or /proc
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.
load: rewrite plugin in Perl.

The old load plugin looked more like an example plugin that a real
one, but since it's one of the most commonly used ones, it's a good
idea to have this _not_ fork.

The rewritten plugin outputs all three load values, as sometimes the
trend with them helps identifying problems. It also supports proper
autoconfiguration (when /proc is not accessible), and dirtyconfig.

The 5-minutes load average is preserved as 'load', but 'load1' and
'load15' are added.
df: improve plugin and merge with df_inode.

The new plugin is simplified and refactored, but also includes support
for graphing inode usage, if linked as df_inode rather than just df.

Additionally, it supports dirtyconfig so that only one execution of
`df` is needed per update.
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.