Accept existing ancestors to new mailboxes

I've been running into this when creating multiple shared mailboxes,
such as shared/Resources/test1@kolab.org, shared/Resources/test2@kolab.org.
After the first mailbox, any subsequent mailbox creation with an @ in
the name would fail (because it tried to recreate the shared mailbox).

The regression was introduced by 31d612831f719cfa288e7e03e51ff553bc9c6a50,
and only affects the localcreate path (seems to work fine in a murder)