Fail to configure home-directory

[Pierre Neidhardt]

I'm hitting issues when setting the home folder.

The default home is /home/ambrevar.  If I change `home-directory' to
"/foo" and run `guix system reconfigure' then "/foo" is created but the
home folder is still "/home/ambrevar".

Furthermore, I cannot mount a filesystem as /home:

```
(file-system
  (device "home")
  (title 'label)
  (mount-point "/home")
  (type "ext4"))
```

Running `guix system reconfigure' hangs after the `usermod...' changes.
Not sure about how to print more debug details.

Did I hit a bug?

--
Pierre Neidhardt

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