[PATCH] udev - activate formt length attribute

We carried the the old callout part selector syntax for two releases
now after it was replaced by the new %c{1} syntax. So here we remove
the old syntax and use the code to possibly specify the maximum count
of chars to insert into the string. It will work with all of our format
chars.
I don't know if somebody will use it, but the code is already there :)

's%3s{vendor}' returns "IBM" now, instead of "IBM-ESXS".

Also added is a test for it and a few words in the man page.