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.
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.