systemd's reboot/shutdown programs don't work under upstart

Bug Description

Installing systemd-sysv replaces upstart's "reboot", "poweroff" and friends. systemd's binaries currently don't know how to talk to upstart to "forward" the request to upstart. This breaks a transition to systemd as default init which doesn't just change the boot= kernel parameter but installs systemd-sysv.

The same problem happens when systemd-sysv is installed but the system gets booted with init=/sbin/upstart.

It would be useful if one could do "initctl reboot", so that there's always a way to reboot the system when using a different init= than /sbin/init.