Make sure we don't fail in device check when using /dev/disk-by-label/*

Make sure we don't fail in device check when using /dev/disk-by-label/*

Quoting Bernhard on IRC:

| i usually use ~by-label/<mysticksname> as device names, so that
| I don't mix up any /dev/sd?? devices and I just realized that
| when i do this (of course I can use ~bylabel/<mysticksname>(:A)
| in the future) grml2usb does neither recognize that my device is
| already mounted (not much of a problem with current kernel
| apparently), but the "superdisk" check which just looks for a
| digit in the last char of the device string, fails as well

Thanks: Bernhard Tittelbach for bugreport and patch