analyze: seperate config file groups by underline

analyze: seperate config file groups by underline

When multiple configuration file groups are shown together (e.g.
systemd-analyze cat-config systemd/system.conf systemd/user.conf), it's nice
to separate them visually.

I tried first to write a line of spaces and underline that. This does not look
too good, because the line is too low. Then I tried a block of blue-background
spaces. In this version, there are two lines, one is full of spaces and
underlined, so visually we get an empty line in the middle.

I then tried underlining the last line of the previous file. This does not look
right, unless the line is full width, i.e. unless spaces are written out until
the end of the line. But when those spaces are added, it's not clear if they
were part of the original file or not. Here, the spaces are between groups, so
it seems less likely that somebody will mistake those spaces for part of the
configuration file.