If nice fails, don't use it to run iperf in linktest.
(Some containers are deprivileged and cannot even run setpriority. Traditional nice is helpful to linktest, because even if its setpriority call fails, it still runs the command. busybox nice just fails (which is arguably the right behavior, but it's not consistent). So now we do a quick trial of the platform nice to see what it can do, and only use it if it succeeds on /bin/true.)
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