[stable6] [backport] Added .ocdata file to check for data folder validity

Backport of #7732 to stable6

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.

Please review/test @icewind1991 @jcfischer @DeepDiver1975 @karlitschek