Udev rules for GUDE DCF77 receivers.

* creates the symlink for /dev/refclock-0 to the TTY your DCF receiver
is connected to.
* set group permission to "ntp" for your DCF receiver's TTY to give
ntpd access to your device.

Supported devices
* GUDE Expert mouseCLOCK USB II

NOTE: You still have to manually adjust ntpd's AppArmor profile to give ntpd
access to your DCF receiver's serial port (%{_sysconfdir}/apparmor.d/tunables/ntpd)