processes: Show real disk IO in addition to process IO (Linux only)

processes: Show real disk IO in addition to process IO (Linux only)

Before this patch, Collectd reports /proc/PID/io fields 'io_rchar/io_wchar' as
'ps_disk_octets' type and 'io_syscr/io_syscw' as 'ps_disk_ops' type.
The names of these types do not match values they represent.

New, correct mapping implemented:

 io_rchar/io_wchar      -> io_octets   (was ps_disk_octets, not used anymore)
 io_syscr/io_syscw      -> io_ops      (was ps_disk_ops, not used anymore)
 read_bytes/write_bytes -> disk_octets (new data collected)

 Closes: #1990