bug#30939: shepherd: detailed output should be placed into well-known location and not tty

[ng0]

Problem, not just when a service is misbehaving after successful system 
reconfigure:

$ sudo herd start smtpd
Password: 
Service smtpd could not be started.
herd: failed to start service smtpd



This is on virtual terminal in X11, as well as in /var/log/messages,
/var/log/shepherd.log, etc.
This is not enough. If I wanted more info, I'd expect that
sudo herd status smtpd would give it (which it does not), so the only
reliable source of information so far is tty 1. Can we fix that in
one of the next shepherd releases? Or is this something we have to
fix in Guix?