Simple Rescue System

[kobliha]

• Implements a rescue system by booting to the Installer Live Media image with special systemd parameters that disable booting to the graphical mode and starting Agama service
• Some services are kept, such as DUD, Self-Update, root password

Problem

The current media do not have a Rescue System but users are asking for it

• https://trello.com/c/Kgaovgu8

Solution

This PR does not implement any special Rescue System. It only reuses what we already have and prevents Agama from starting.

Testing

• Parameters were tested manually on a Kernel command line

[imobachgs]

It looks fine. Did you try it?

Please, update the changes file.

[kobliha]

I tried that just manually on a Kernel command-line

Discussing on IRC about whether it will work.

[lslezak]

Looks good as the initial implementation for some rescue system.

However there are still some questions:

• Which services should be actually started in the rescue mode? Should we e.g. configure the network?
• Systemd already provides it's own rescue mode, boot with systemd.unit=rescue.target live.password=password (The rescue target requires a configured root password, that's why I updated the live-password.service file in this PR to allow setting the password in the rescue mode as well.)
• Implementation detail: Maybe we should define a separate systemd target for this?

[kobliha]

Thanks @lslezak, yes I thought about a special target too, but it's a bigger thing and requires a better plan, more impl. time and better testing. I simply went for the simplest solution as we don't have the requirements yet. Anyway, you should open an issue or a discussion here at GH for the topic.