adduser: create dataset only if home is directly within dataset

Currently, if the prefix of the new home directory is a subdirectory
of a ZFS dataset, adduser will create a new dataset up one or more
levels from the intended destination. "pw useradd" will then create
a normal directory in the desired location, leaving an unused dataset.
Check for this situation when determining whether to create a dataset,
and let pw create the directory.

Reviewed by: des
Differential Revision: https://reviews.freebsd.org/D45229
MFC after: 3 days