Trace mode slow? Test this patch.

[Clemens Lang]

Hi developers,

I recently noticed that trace mode was significantly slower on modern
macOS than it used to be, and found some time to investigate the root
cause.

If you saw the same, please test the current master branch state with
  https://github.com/macports/macports-base/commit/3b949ccfe7a309bce1cc7041cdf56871890b784d
It should return your trace mode to previous speeds (about a 6-8x
improvement compared to how slow it currently is).

I've also done some benchmarking and have a few more patches in my
pipeline that should bring a bit more (but rather minor compared to
this) improvements.

HTH,
Clemens

P.S.: I have not reported this problem to Apple. If somebody wants to
verify that it occurs with all uses of copyfile(3), it would be nice if
they could let them now.