ls needs perl's -w to warn of so many bad switch combos

[Dan Jacobson]

True, the man pages says "-r, --reverse reverse order while sorting",
and we are not sorting, but still, can't we add something like perl's
-w, to tell the user "-r is useless with -U.  ignoring -r"

$ /bin/ls -U|sum
58867   109
$ /bin/ls -Ur|sum
58867   109
$ /bin/ls -Ur|tac|sum
14097   109

There are tons of other ls switch combos in which one switch is
silently ignored also.  Down with silence.  OK, compromise: on the man
page list what really happens for various switch combos.
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