o Implement the stop_boot subroutine [1]. This subroutine can be used by

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