Nvim note: intentionally did not include `--ttyfail` since its purpose
is not clear. (And it isn't used in any Vim test files/scripts).
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Problem: When the input or output is not a tty Vim appears to hang.
Solution: Add the --ttyfail argument. Also add the "ttyin" and "ttyout"
features to be able to check in Vim script.
2cab0e1910