We still need to set the hooks to prevent improper manipulations thereafter
at the loader prompt, but as it is we're actively preventing loaddev from
being set correctly in some circumstances (ZFS in particular) and doing more
work than needed with currdev -- that hook in particular validates it as a
correct device, which we can assume isn't needed in this context.

Reviewed by:	imp, sigsys@gmail.com
Submitted/Diagnosed by:	sigsys@gmail.com
Differential Revision:	https://reviews.freebsd.org/D23390