procfs: create an aggregated plugin for all data coming from /proc/stat

The list of plugins currently merged in is:

 - cpu;
 - forks;
 - interrupts;
 - swap.

The new plugin, written in Perl, leverages multigraph so that
/proc/stat is only accessed once, and instead of having multiple awk
calls among different plugins getting the same data.

Additional changes to the graph compared to the previous plugins:

 * the cpu graph now output the guest_nice data (which was added to
   the kernel back in 2009);
 * the swaps graph is only generated if /proc/swaps is present as
   not everybody has swaps enabled at all.
plugins: start implementing a new framework for plugins writing.

Munin::Plugin::Framework allows a much easier way to write new plugins
by abstracting the underlying protocol between node and master.

The idea is to have the plugins only declare what they want to have
displayed, eventually merging the code for configuration and
value-setting, and leaving it to the framework to generate the
configuration and fetch output.