Mostly for debugging purposes, because I'm tired of having to use
'setsid' to force Pageant to select the GUI passphrase prompt when I'm
trying to fix bugs in gtkask.c. But I can also imagine situations in
which the ability to force a GUI prompt window might be useful to end
users, for example if the process does _technically_ have a
controlling terminal but it's not a user-visible one (say, in the back
end of some automation tool like expect(1)).
For symmetry, I also provide an option to force the tty prompt. That's
less obviously useful, because that's already the preferred prompt
type when both methods are available - so the only use for it would be
if you wanted to ensure that Pageant didn't _accidentally_ try to
launch a GUI prompt, and aborted with an error if it couldn't use a
tty prompt.
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