Added .ocdata file to check for data folder validity

In environments where the data folder is mount from another partition,
it is important to check that the data folder we see is actually the
real one. If the mount failed for some reasons, this fix will make
ownCloud temporarily unavailable instead of causing unpredictable
behavior.

Fixes #7488 

Please review/test @jcfischer @DeepDiver1975 @karlitschek 

#### To test:
- [x] Initial ".ocdata" file creation on setup
- [ ] ".ocdata" file creation after upgrade

#### For each of these tests, check that the sync client goes to "service unavailable mode" and the web UI shows an error message:
- [x] Manually remove ".ocdata" file to simulate the issue, then recreate it.
- [ ] Test with real FS mount on "data" dir: umount, check, then remount
- [ ] Test when "data" dir is mounted in a subdir of a real FS mount: umount, check, then remount.
- [x] ./occ shows error when ".ocdata" missing