processes plugin: Add collection of IO-metrics.

processes plugin: Add collection of IO-metrics.

I attach a patch for collectd-4.8.0 adding a new feature which read IO
data for a process using /proc/<pid>/io (only works on linux > 2.6.20),
it's very usefull to monitoring disk throughput and what process is
related with high disk IO. I'm using this patchs for days in some
database environments with good results, but I think that is not enough
stable yet (more testing is required). The patch read from
/proc/<pid>/io (where available) the data for syscr and syscw (read and
write operations) and rchar and wchar (read and write bytes).