[PATCH] udev - switch callout part selector to {attribute}

[PATCH] udev - switch callout part selector to {attribute}

Here we change the magic callout part number selector to the new
atribute syntax. The syntax to select the second part of the callout string:

 '%2c' is now '%c{2}'

I think it's more clear and we no longer misuse the length argument.

The old syntax is still supported, but we should remove it some
time in the future.