bug#29185: Dired: don't just say "by date" when -tc is used

[積丹尼 Dan Jacobson]

"s runs the command dired-sort-toggle-or-edit ...
With a prefix argument, edit the current listing switches instead."

OK I added S. Now the mode line shows "(Dired -aloS)". Good.

Now instead I added -tc. But the mode line just says

However the sorting order is very much different than the usual "(Dired
by date)" (just -t, not -tc).

Therefore the modeline should say "(Dired -alotc)", not "(Dired by
date)".

You might say "But it is still a date-based sorting method".

Yes, but unlike -r, -c is not just a different sorting direction...

Or, always say "(Dired by date -xyz)" showing all the switches all the
time.

Also maybe don't just say the same "by name" for -lc vs. -l...