scripts in rc.d to stop rc(8) from booting into multi-user mode when
a critical or severe error condition is encountered.
o Modify scripts in etc/rc.d that already implemented this functionality
independently.
o Document it.
[1] - This subroutine was implemented in FreeBSD in rc.d/fsck. I moved it
to rc.subr(8). Our version differs slightly in that it takes an
optional argument to stop the boot even if "autoboot" is not set.
Obtained from: NetBSD
MFC after: 2 weeks
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