systemctl: don't search in the full argv[0] for the invocation name

argv[0] might be prefixed by a path, and we shouldn't get confused by
that. Hence provide a simple helper call that abstracts the checking
away, which we can use everywhere, and expose the same behaviour, even
if argv[0] is not set.

(While we are at it, port all other multi-call binaries over to the new
helper, too)

Follow-up for: d41a9e4f