reboot: Default to `--check-inhibitors=yes`

[cgwalters]

See all the discussion previously in coreos/rpm-ostree#2862

We should do this here too. Note that the default upstream I think is changing to do this by default, but we should opt in now.

Also tangentially related to reboots there's a DesignatedMaintenanceTime in newer systemd that we can encourage people to use. https://github.com/systemd/systemd/releases/tag/v257