bug#55335: openssh-service no longer listens on IPv6

[Christopher Baines]

This looks to be a recent regression, probably connected with the
shepherd now doing the listening, rather than sshd itself.

Previously, you could use both IPv4 and IPv6.

  netstat -tlnp | grep sshd
  tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN    
  26683/sshd: /gnu/st
  tcp6       0      0 :::22                   :::*                    LISTEN    
  26683/sshd: /gnu/st

Now though, it looks like with shepherd doing the listening, you can
only use IPv4.

  netstat -tlnp | grep 22
  tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN    
  1/guile


On an affected machine, you can reproduce this by trying to SSH over v6.

  cbaines@lakeside ~$ ssh 127.0.0.1
  The authenticity of host '127.0.0.1 (127.0.0.1)' can't be established.
  ED25519 key fingerprint is SHA256:1wV7w84awrGv5ilP5e8k5ygIvSkXSJ6LIy3MnqZG2Jw.
  This key is not known by any other names
  Are you sure you want to continue connecting (yes/no/[fingerprint])? ^C

  cbaines@lakeside ~$ ssh ::1
  ssh: connect to host ::1 port 22: Connection refused


This isn't an issue if you're not using IPv6, but if you have a machine
only accessible via IPv6, then you can't ssh in. The main workaround
I've found is getting access via other means, then starting sshd
listening on a different port (as the shepherd is using 22).

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