Avoid creating a "shared" toplevel folder

It used to be that ancestors in a hierarch were not crated, but that
changed in https://git.kolab.org/R111:31d612831f719cfa288e7e03e51ff553bc9c6a50

Because of that the first shared folder that is created results in the
shared/ folder being claimed by the backend creating the folder.
Subsequent folders created on other backends will fail, because the
second backend will also attempt to create shared/, which will fail to
be reserved on the master since it already exists.

This patch introduces a hardcoded excemption for the toplevel shared/
folder to not be created, so we can continue to create shared folders
under the shared/ toplevel folder while having the folders distributed
over backends.