systemctl: tweak output of Listen: fields a bit

It's probably a good idea to minimize the number of field names to show
in the "systemctl status" output, in order to make them useful as a
guide for the reader how things are "grouped". This patch moves
information about the used socket technology to the end of the output
lines in brackets, rather than into the field names. This turns the used
socket technology into what it is -- peripheral meta information --
instead of something that was at the core.

New output:

systemd-journald.socket - Journal Socket
         Loaded: loaded (/usr/lib/systemd/system/systemd-journald.socket; static)
         Active: active (running) since Fr 2013-03-29 02:16:30 CET; 1 weeks 0 days ago
           Docs: man:systemd-journald.service(8)
                 man:journald.conf(5)
                 man:systemd-journald.service(8)
                 man:journald.conf(5)
         Listen: /run/systemd/journal/stdout (Stream)
                 /run/systemd/journal/socket (Datagram)
                 /dev/log (Datagram)
systemctl: tweak drop-in display

So far we didn't place spaces between the box drawing chars and the
values next to them. Let's be consistent here.

(Or to turn this around: if we really want to place a space there we
probably should do that in all our tree outputs, not just here...)