This is the easiest place to implement _something_ that will work as a
runtime passphrase prompt, which means I get to use it to test the
code I'm about to add to the Pageant core to make use of those
prompts. Once that's working, we can think about adding prompts for
the 'proper' usage modes.
The debug-mode passphrase prompts are implemented by simply reading
from standard input, having emitted a log message mentioning that a
prompt is impending. We put standard input into non-echoing mode, but
otherwise don't print any visible prompt (because standard output will
in general receive further log messages, which would break it anyway).
This is only just good enough for initial testing. In particular, it
won't cope if two prompts are in flight at the same time. But good
enough for initial testing is better than nothing!
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