send notifications for timed shutdowns before 10 minute mark

[mbiebl]

Originally filed as downstream bug report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=884994

It appears that for timed shutdowns we only start sendition wall messages 10 mins before the actual shutdown. We do that every minute then.

sysvinit was a bit more sophisticated in that case:

From be the needwarning() function in sysvinit's
src/shutdown.c[1]. With the default setting QUIET_NONE you get:

• a warning every minute in the last 10 minutes
• a warning every 15 minutes in the last hour
• a warning every 30 minutes in the last 3 hours
• a warning every hour before that

It would make sense to implement something similar for systemd, to give users more advance warning.

[1] https://sources.debian.org/src/sysvinit/2.93-8/src/shutdown.c/#L467