bus: Add LSM label to connection loginfo string

bus: Add LSM label to connection loginfo string

If it is set (i.e. if an LSM is in use) this will make it appear in
various places in log output.

With SELinux, for example, this appends something like:
   label="system_u:object_r:unlabeled_t:s0"

This commit partially rearranges the code which sets the loginfo string,
so that it consistently puts a space between fields, and not one at the
end.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=68212
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>