One more default sorting order ("grouping").

One more default sorting order ("grouping").

If --group-by is not set, output will be grouped
by month by default (as long as --boundary is
not set to 'level' or 'average', where grouping
by newsgroup is default).

Now we default to 'newsgroup' if just one newsgroup
is requested by --newsgroups, but more than one
month by --month.

Both defaults can be overridden.

But forced --group-by=month for --report type
'average' or 'sum' in front so defaults are
not checked.

Signed-off-by: Thomas Hochstein <thh@inter.net>