WISH: user controlled stderr output

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Being able to redirect output to stderr was a long standing wish.

https://stackoverflow.com/questions/27913863/output-to-stderr-in-rebol2
https://www.mail-archive.com/rebolist@rebol.net/msg03802.html

There is also an issue #1862 requesting that errors are automatically redirected to stderr.

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Here is required change for a posix systems: Oldes/Rebol3@9c40364

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So far it is implemented as:

print 1 ;<- std_out
modify system/ports/input 'error on
print 2 ;<- std_err
modify system/ports/input 'error off
print 3 ;<- std_out

The name of the port is bad... as it is not just input, but also output.

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Any idea for name changing or functionality change is welcome! This work was so far done just as a step to implement this: #1862