Comment 7 for bug 1206164

While these days the systemd based timesync* things are doing most of the work there is still a lot of buzz around the automated ntpdate on ifup. I'll try to to summarize the outcome of multiple discussions and bugs around this and propose a solution.

- I've seen various reports of ntpdate syncing too much or time changes due to that annoying people.
- but then there are different user scenarios so some want and others won't "autoupdate" feature on ifup
- the ones that want it disabled are usually only "inconvenience issues" like too much calls, but also cases like "know it will hang", so could I disable it in advance
- the ones require the updates are mostly having more severe issues (like breaking authentication due to time being off afterwards)

We should try to create a solution for both parties to be able to config the system to their way.
So lets keep the default to sync (as it seems the more critical way), but provide a way to disable it via the config files.

Doing it via an environment variable also allows to "overwrite" it in ifup calls like
  DISABLE_NTPDATE=1 ifup eth0

The config file has to be read in ntpdate-debian if the variable is not set.

Then just a simple check for that variable in the script to exit if disabled:
[ "${DISABLE_NTPDATE:-0}" != "0" ] && exit 0

So the behavior would be like:
1. default it is running on ifup
2. one could set a different default in the config file
3. one can overwrite whatever was set via an environment variable

... creating a patch for that and providing to Debian for a latter sync.