feat [close #310]: Mounts system mountpoints like /home and /etc before systemd starts

[taukakao]

Fixes #310

This command replaces the systemd mount units with a command that has to be run before systemd.
This ensures that systemd recognizes the correct state of /etc.
This also makes the system more predictable, since there's not two different /etc folders to worry about.

To migrate to this version, the init system has to call the two new abroot functions before executing systemd.
The systemd mount units of old abroot versions don't cause any problems, since systemd doesn't remount them, so no futher migration is needed.

[taukakao]

Also, I can squash the commits if it's necessary but I think these commits are already pretty atomic.

[matbme]

Implementation looks good. My spam of suggestions is mostly related to missing translations.

[mirkobrombin]

LGTM. I agree with Mat about the missing translations, but per our conversation on discord, should be fine due to the command being strictly internal.

Anyway, adding the translation keys should not be so hard, so I'll leave the decision to you.

[taukakao]

As discussed on Discord, I don't want to put translations in.
These commands are not user-facing so they don't need to be translated. It would only put extra work on the translators without making a difference.

[mirkobrombin]

Not to go over Mat but given the discussions on Discord regarding the translations and given the resolution of the discussions, I still proceed with the merge without his confirmation.