cgls: show delegation boundaries by underlining the cgroup in the output

cgls: show delegation boundaries by underlining the cgroup in the output

This should help visualize where one manager's territory begins and
another's starts. Do this by underlining (since it's a "cut" point an
underline made most sense to me). Since underlining is not visible on
the console let's also show an ellipses for all lines that are
delegation boundaries.

Unfortunately this all is not as useful as it appears. The
"trusted.delegate" xattr is only visible to roo, which means
"systemd-cgls" has be called as root to show the boundaries.
Unfortunately cgroupfs doesn't support unprivileged xattrs on cgroups.