#11 Distinguish between device ID, label, subfunction

[Christian]

For a device, distinguish between:
- the device ID: used to identify a device in the config file
- the label: name of the device that is displayed (max. 3 characters)
- the subfunction: e.g. for function "cpu" cpu0/cpu1 in case there are several CPUs available or for function "io" hda/hdb/sda for the several disks (actually, the subfunction is the string for which to search stat files)

In 1.0.x versions, the device ID was missing (given indirect by the sequence in the config file) and the lable was used in some cases at the same time as subfunction (e.g. in case of the function "io" or "net").