--user basically gives messages from your own systemd --user services.
--system basically gives messages from PID 1, kernel, and --system
services. Those two options are not exahustive, because a priviledged
user might be able to see messages from other users, and they will not
be shown with either or both of those flags.
Split the large bash completion script into separate, smaller files each
named after the binary it is used for and move the files to
/usr/share/bash-completion/completions. This way the completions can be
loaded on demand and we only install the completions for the tools we
actually build. The old path /etc/bash_completion.d/ is deprecated and
will disappear in the future.