procfs: create a single 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.