daemons: Support --timeout 0

daemons: Support --timeout 0

When using systemd we want to take advantage of socket activation
instead of keeping daemons running all the time, so we default to
shutting them down after two minutes of inactivity.

At the same time, we want it to be possible for the admin to opt
out of this behavior and disable timeouts entirely. A very natural
way to do so would be to specify a zero-length timeout, but that's
currently not accepted by the command line parser. Address that.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>