Use telinit to detect upstart version. PUP-2790

Unlike initctl, it is not replaced by a dummy script during installations.
I should explain a bit more, I think:
Only initctl is diverted during installation. The script that replaces it is basically a NOOP (except for a message it displays). telinit is not replaced. 
This means that the correct init system can be detected using telinit, but one should still use initctl to start/stop services (which includes some other commands that are symlinks to initctl in the upstart packages), which ensures that they are not accidentally started during a system installation.