HBSD: prevent init hijacking if PAX_HARDENING enabled

init hijacking steps:

user$ cp /sbin/init /usr/home/user/init # or just create an initit compatible exploit
ctrl + alt + del
freebsd-loader: ESC
freebsd-loader: set init_path="/usr/home/user/init"
freebsd-loader: boot

and voila, the system booted fine with modified init, even if enabled
root password checking to system console in single user mode.

Probably the modified init could touch sensitive data (master.passwd, ...)

github-issue: #28
Signed-off-by: Oliver Pinter <oliver.pinter@hardenedbsd.org>