bug#81311: headers for comm, wc

I'm saying
$ comm --add-header a b
should add a first line:
Only in a Only in b In both

Well it looks rather ugly, so maybe

Only in a
         Only in b
                   In both

Anyways, the idea is we are making a table,
and tables need a header to make sense.

Sure, we can make our own header, but there should be a basic header
available too.

Hmm, same problem with the wc(1) command.