units: wait for plymouth to shut down in rescue.sevice (systemd#3367)

In the same vein as commit ac59f0c which added
the --wait option to the emergency service, this patch makes sure that plymouth
has exited before entering into the rescue mode.

Author: fbuihuu

units/rescue.service.in

@@ -16,7 +16,7 @@ Before=shutdown.target
 [Service]
 Environment=HOME=/root
 WorkingDirectory=-/root
-ExecStartPre=-/bin/plymouth quit
+ExecStartPre=-/bin/plymouth --wait quit
 ExecStartPre=-/bin/echo -e 'Welcome to rescue mode! After logging in, type "journalctl -xb" to view\\nsystem logs, "systemctl reboot" to reboot, "systemctl default" or ^D to\\nboot into default mode.'
 ExecStart=-/bin/sh -c "@SULOGIN@; @SYSTEMCTL@ --job-mode=fail --no-block default"
 Type=idle