Add support for %{stderr} and %{stdout} for --write-out

[gevaerts]

Note that the actual stderr test is incomplete: runtests.pl doesn't have easy support for stderr

[jay]

I proposed something similar a while ago but it died due to lack of interest, see https://github.com/curl/curl/compare/master...jay:add_flags_to_write-out. Yours is cleaner. In any case one issue is stderr is used for errors and that could disrupt the write-out text. I liked fd 3 - 9 as an alternative even though I couldn't get it working in Windows.

[gevaerts]

My approach is mainly cleaner because it does a lot less... I do like the flexibility of your approach though, and separating output variables from things like this does make sense.

[bagder]

I prefer this version due to its simplicity and minimalism.

[bagder]

Would you mind also removing the TODO entry for this? (18.11)

[gevaerts]

OK, should be done now! (as well as rebased)

[bagder]

Thanks!