Revert "units: don't kill the emergency shell when sysinit.target is triggered (...

Revert "units: don't kill the emergency shell when sysinit.target is triggered (#6765)" (#6904)

This reverts commit f1e24a259ca182b6cd8a723a56da43435ce48aac.  Oops.

# systemctl emergency
Failed to start emergency.target: Transaction order is cyclic. See syste...
See system logs and 'systemctl status emergency.target' for details.
# systemctl status emergency.target
● emergency.target - Emergency Mode
   Loaded: loaded (/usr/lib/systemd/system/emergency.target; static; vendor preset: disabled)
   Active: inactive (dead) since Mon 2017-09-25 10:43:02 BST; 2h 42min ago
     Docs: man:systemd.special(7)

systemd[1]: sysinit.target: Found dependency on sysinit.target/stop
sysinit.target: Unable to break cycle starting with sysinit.target/stop
network.target: Found ordering cycle on wpa_supplicant.service/stop
network.target: Found dependency on sysinit.target/stop
network.target: Found dependency on emergency.target/start
network.target: Found dependency on emergency.service/start
network.target: Found dependency on serial-getty@ttyS0.service/stop
network.target: Found dependency on systemd-user-sessions.service/stop
network.target: Found dependency on network.target/stop
network.target: Unable to break cycle starting with network.target/stop

IMO #6509 is ugly enough that we should aim to answer it.  But it could
take some time to investigate, so let's re-open the issue as a first step.