Stay alive during shutdown

As we are not PID 1, upstart SIGTERMs everything after calling
poweroff/reboot, including systemd-shim. But we need to stay alive to keep the
in_shutdown global state, so that we can disable suspends/hibernate during
shutdown. So as soon as poweroff or reboot gets requested, add ourselves to
/run/sendsigs.omit.d to stay alive.

https://launchpad.net/bugs/1211514