* by default, warning messages go to stderr
* if -A or -a is used, send warning messages to stdout instead.
The behaviour in 1.50 meant that
somevar=`sysctl -n machdep.booted_kernel 2>/dev/null`
could end up with $somevar="machdep.booted_kernel: the value is not available"
rather than the more useful $somevar="".
(This way, "sysctl -A | grep ..." is still useful.)
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