The docstring for dired-guess-default claims it should handle multiple
instances of COMMAND each of which could be a STRING or a FUNCTION which
evaluates to a STRING ...

The attached patch does that. It also weeds out strings that don't represent
valid programs on the host machine.

It turned out that function dired-guess-shell-command wasn't ready for
the multiple responses, so the patch corrects that as well.

My package diredc has an additional feature of an option for a universal
fall-back. My memory is that the emacs developers rejected the idea when
I originally proposed it, so it's not in this patch.

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