journal: timestamp support on console messages

journald mimics the kernel here: timestamps will be printed if
/sys/module/printk/parameters/time contains "Y".
man: add bootctl(8)

https://bugzilla.redhat.com/show_bug.cgi?id=1014303
zsh-completion: add bootctl
Resolve /dev/console to the active tty instead of just "tty0"

When resolving /dev/console one would often get "tty0" meaning the active VT.
Resolving to the actual tty (e.g. "tty1") will notably help on boot when
determining whether or not PID1 can output to the console.
Only disable output on console during boot if needed

If there are no more jobs on console, no need/we shouldn't disable output.
Fix possible lack of status messages on shutdown/reboot

Since 31a7eb86 the output on console can be disabled to avoid colliding with
gettys. However, it could also lead to a lack of messages during
shutdown/reboot.