Finally encrypt subdirectories during sync

Finally encrypt subdirectories during sync

We catch when a directory is inside a known encrypted folder and in such
a case we now do the following:
 1) we encrypt the folder meta data (its name) properly and create it
    under that mangled name on the server side
 2) we mark the new folder itself as encrypted

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>