systemctl: ignore shutdown's "-t" argument (#7459)

systemctl: ignore shutdown's "-t" argument (#7459)

We should not only ignore "-t" itself, but also whatever is passed to
it.

This pretty much reverts the core of
a4420f7b8ed73b05ef6f31622101e7804daef69f, and adds back in the status
quo ante. What a difference a ':' can make.

This also adds a quick comment for this, so that we don't make this
mistake again.

Fixes: #7413