libbe: avoid copying encryption-related props

libzfs insists that these be cloned from the origin, so avoid making a
deep copy of them ourselves to unbreak creating a new BE from a BE with
encrypted components -- in today's environment, without a loader that
does encryption, this means a deep BE setup where something underneath
the BE (e.g., home directories) are encrypted.

Reported and tested by: arrowd
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D48464