uuidd: add cont_clock persistence

uuidd: add cont_clock persistence

cont_clock requires a correct time setup and therefore it
must be possible to detect a step back between uuidd starts.

Reserving the next 10 seconds is sufficient and should not
have a noticeable performance impact. It will also provide
the possibility to start with the clock_reg from the previous
session when the system was rebooted.

Whith that, the early cont_clock initialization in uuidd
should be removed because writing the cont_clock persitence
when -C was not set is useless and might be confusing.