Disable hwclock-save.service on Jessie

systemd (215-13)’s debian/changelog entry says:

   * Add hwclock-save.service to sync the system clock to the hardware clock on
     shutdown, to provide monotonic time for reboots. (Note: this is a hack for
     jessie; the next Debian release will enable timesyncd by default).
     (Closes: #755722)

We need to disable that, as per our design goal to avoid leaving traces on the hardware being used, as much as possible.

For more information, see https://bugs.debian.org/755722. (And interestingly, there we learn that under sysvinit (Wheezy), util-linux’s hwclock.sh is supposed to “overwrite the RTC with the system clock partway through the shutdown”, so that’s not exactly a regression against Tails/Wheezy, but rather a problem we already had. Will file a dedicated ticket about it.)

Related issues

• Related to #9364 (closed)

Original created by @intrigeri on 9363 (Redmine)