Revert "benchmark - slightly better stderr"

This partially reverts commit 5ab6db2607.

The change of argument --threads N doesn't just change output, it
changes the mode of operation.
With --threads 0, the benchmark runs wholly in the main thread.
With --threads 1, it starts 1 worker thread, which does the work, and
the main thread measures how long it takes to complete.

benchmark/run

@@ -10,7 +10,7 @@ function run {
     local threads="$2"
     local iters="$3"
     shift 3
-    $runner --threads 1 --iterations $iters "$@"
+    $runner --threads 0 --iterations $iters "$@"
     if test $threads -gt 0; then
         $runner --threads $threads --iterations $((iters/threads)) "$@"
     fi