document how to switch to single-user mode

[anarcat]

Before Debian switched to systemd, shutdown now would reset the system into
single user mode, doing roughly the equivalent of telinit 1.

Now, systemd's shutdown command does not behave that way; it defaults to
poweroff which might be confusing for users (like me) used to the previous
method.

Because I don't use the command often, I keep being stumped by this behavior,
and every time I look at the shutdown(1) manpage, I don't understand why I
can't find what I am looking for. This patch should make sure that people like
me find their way back to some sort of reason.

Maybe the proper way to fix this would be to restore the more classic
behavior, but I'm definitely not going to climb that hill. Besides, I clearly
remember the time I found out about the shutdown command and was really
confused when it brought me back to a command-line prompt. That was really
counter-intuitive and I find that change to actually be a good thing. So I'm
not proposing to change this behavior, merely document it better.

I originally added this to the -P option but it was suggested adding a new
COMPATIBILITY section instead, where other such issues could be added.

The COMPATIBILITY section is not actually officially documented. man(1)
talks about a CONFORMING TO section, but shutdown(1) is not
POSIX (shutdown(2) is, of course), so there's no actual standard on how this
should work.

The other option I considered was to add a BUGS section, but that seemed to
inflammatory, and definitely counter-productive.

[anarcat]

Before Debian switched to systemd, shutdown now would reset the system into single user mode, doing roughly the equivalent of telinit 1.

If people are doubting this, their memory, or mine, they are welcome to review the sysvinit shutdown manpage:

https://manpages.debian.org/bullseye/sysvinit-core/shutdown.8.en.html

which explicitly says:

shutdown does its job by signalling the init process, asking it to change the runlevel. Runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel 1 is used to put to system into a state where administrative tasks can be performed; this is the default if neither the -h or -r flag is given to shutdown.