Use the upstream (but patched) sysinit.target
Install {rescue,emergency}.{target,service}

Also, symlink kbrequest.target to rescue.target as suggested by the
systemd.special manpage.  This way, you can start a sulogin rescue
shell by pressing Alt+Up.
Remove handling of "debug2" and "S|s|single" kernel command line options

The "S|s|single" option is handled by systemd (starting
rescue.target).  And the rescue target basically removes the need for
a special debug shell.  (Also, there is "systemd.crash_shell=1" for
starting a shell if systemd crashes.)