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Although it's never been officially documented, RISC OS has historically
used the following values for R0-R2 when calling the exit environment
handler:

R0 = 0 (or some other "safe" value)
R1 = R1 from OS_Exit
R2 = Return code

Some exit handlers were relying on this (e.g. Shared C Library /
ANSILib). Changing OS_Exit to use TaskControl_ResetStacks was causing R1
& R2 to be corrupt, with the result that non-zero, non-error-generating
return codes from C programs were being lost and reset to zero.

Document things more clearly and ensure R0-R2 have acceptable values so
that software which relies on the old behaviour will work again.