systemd-inhibit: ignore signal interrupt from keyboard (#8569)

By default both processes, systemd-inhibit and the forked one, receive
the signals. Pressing Ctrl+C on the keyboard results in SIGINT being
sent to the processes, followed by SIGTERM being sent to the forked
process when systemd-inhibit exits. This can cause trouble when the
forked process does not clean up properly but exit immediately.

Instead make systemd-inhibit ignore SIGINT, leaving it to the forked
process to clean up and exit.