Save and restore the data toggle value when a pipe to an endpoint

is closed and then reopened.  This may be necessary now that
we no longer clear endpoint stalls every time a pipe is opened.
Previously we could assume an initial toggle value of zero because
a clear-stall operation resets the device's toggle state.

Derived from work in FreeBSD.

This is most likely to affect devices like printers, which open
pipes for short periods of time and close them again.

ok dlg@

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