Exact division

[Niels Möller]

With bdiv_q now working, I wrote a simple divexact function, and I use
it in mpn/gcdext.c where it needs to compute the second cofactor. As a
result, the GCDEXT_DC_THRESHOLD on shell was reduced from 496 to around
415. I guess it would be reduced a little more more with a more clever
bidirectional divexact.

(There are two problems with mpn/divexact.c: (1) Torbjörn doesn't trust
the current implementation, and (2) it requires that the divisor is
odd. Feel free to move the code in mpn/gcdext.c:divexact).

Regards,
/Niels

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