bsdinstall: improve zfsboots ability to handle datasets under a BE

[brd]

Summary

Currently we only set canmount=noauto on the root of the BE, this
change makes it so we set it on all datasets under the BE as well.

[allanjude]

Yeah, this make sense. We might also need to double check that bectl does the thing thing here when you clone the deep BE

Reviewed by: allanjude