add bash completion for systemctl --system

I've been playing recently with systemd on Arch, and had much fun. But
soon, alas, my fingers started to ache from repeatedly writing
systemctl restart some-long-service.service. So, I wrote a completion
script. I figured other people may want to use it, so I prepared a
patch against systemd-git (attached).

There are some notes/disclaimers, however:

- It requires bash>=4.0, sed, grep and awk. A bash-completion package
is not strictly needed; sourcing the file is enough.
- It wouldn't work properly with --session, as I had no way to test it.
- It uses the output of systemctl list-units directly when that's
enough, but also runs systemctl show when completing on some verbs
(for example, to check for AllowIsolate=yes). This /may/ be somewhat
slow once there are many units, since it calls a dbus method on each
one. Is there a faster way to have that information?
- The code is perhaps a bit long and messy; honestly, I blame the tool ;)

One way to improve on the situation is to integrate some completion
code in systemctl itself, the way e.g. gdbus, gsettings and django do
it. This will allow for finer grained and faster completions, and it
won't be necessary to keep the verb/option tables in sync with some
other file. But it does mean adding all of this code in C. If this is
acceptable, I'll try to have a go at it.

Finally, a couple of completion tips I run into:
- If you alias systemctl to, say, sctl, you get completions on that
too by running to following command:
complete -F _systemctl sctl
- Add the following line to your .inputrc, to have the completion show
after only a single tab press:
set show-all-if-ambiguous on
It makes the shell quite more pleasant.

Hope it's good enough!

Ran

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