fs: use dup/dup2 for stdio hide and restore

[sreimers]

No description provided.

[alfredh]

I tested with retest and the "OK" is not printed:

$ ./retest -r
retest: libre version 2.9.0 (x86_64/Darwin)
using async polling method 'kqueue'
regular tests:       Alfreds-MBP:retest alfredh$ 

is the restore function working ?

[sreimers]

Thanks for testing on macOS, will try to debug, on Linux it works fine and better than freopen. Needs more testing on Windows too.

[sreimers]

ok, retest restore is broken on linux too, so I think we have to fix the usage in retest. Simple usage like here https://github.com/baresip/baresip/blob/main/modules/portaudio/portaudio.c#L306-L308 works fine.

[alfredh]

this one is good to go