* Only toggle nightmode via the grayscale fb info flag on Kindle
Most NTX boards require elaborate trickery (see fbdepth) to actually
send a vput ioctl without the driver screwing with the rotation behind
your back.
This was causing spurious hardware rotations on exit on most Kobo
devices, something which might have gone mostly unnoticed, as current
Nickel versions will sanitize it on startup.
A simple repro was instead to start an USBMS session, as that exits
KOReader and restarts it without going through fbdepth again ;).
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