Add new reboot and system-upgrade modes
Some themes show certain text strings to the user depending on the mode, see e.g. the shutdown vs reboot mockups of: https://wiki.gnome.org/Design/OS/BootProgress
Besides during shutdown vs reboot, we also want different theming for installing offline (security) updates versus doing an offline OS upgrade.
To make this possible this commit adds new reboot and system-upgrade modes which can be specified either when starting plymouthd, or through plymouth change-mode --.