sulogin-shell: Use force if SYSTEMD_SULOGIN_FORCE set

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