sulogin-shell: Use force if SYSTEMD_SULOGIN_FORCE set

When the root account is locked sulogin will either inform you of
this and not allow you in or if --force is used it will hand
you passwordless root (if using a recent enough version of util-linux).

Not being allowed a shell is ofcourse inconvenient, but at the same
time handing out passwordless root unconditionally is probably not
a good idea everywhere.

This patch thus allows to control which behaviour you want by
setting the SYSTEMD_SULOGIN_FORCE environment variable to true
or false to control the behaviour, eg. via adding this to
'systemctl edit rescue.service' (or emergency.service):

[Service]
Environment=SYSTEMD_SULOGIN_FORCE=1

Distributions who used locked root accounts and want the passwordless
behaviour could thus simply drop in the override file in
/etc/systemd/system/rescue.service.d/override.conf

Fixes: #7115
Addresses: https://bugs.debian.org/802211

docs/ENVIRONMENT.md

@@ -112,6 +112,12 @@ systemd-timedated:
   first existing unit listed in the environment variable, and
   `timedatectl set-ntp off` disables and stops all listed units.
 
+systemd-sulogin-shell:
+
+* `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
+  root password is not available (such as when the root account is locked).
+  See `sulogin(8)` for more details.
+
 bootctl and other tools that access the EFI System Partition (ESP):
 
 * `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are