Enable bsdinstall hardening options by default.

As discussed previously, in order to introduce new OS hardening
defaults, we've added them to bsdinstall in 'off by default' mode.
It has been there for a while, so the next step is to change them
to 'on by defaul' mode, so that in future we could simply enable
them in base OS.

Reviewed by: brd
Approved by: adrian
Differential Revision: https://reviews.freebsd.org/D9641