Ensure the volume path is '/' terminated and allow nested volumes

Ensure the volume path is '/' terminated and allow nested volumes

The previos change to the way checking for duplicate and nested paths only
works if the path is terminated with a '/'.
Allow nested volumes. This was working in 2.2 and can be used eg for sharing
a ZFS snapshot directory of a volume. Afaict it only works with sane AFP
semantics by adding 'cnid sheme = tdb' to the nested volume which also
makes it read-only and by adding the name of the nested directory to
the veto name list.