Divides a by b and returns the integer result. The result rounding is defined by the round, which can have the following values:
GMP_ROUND_ZERO: The result is truncated towards 0.
GMP_ROUND_PLUSINF: The result is rounded towards +infinity.
GMP_ROUND_MINUSINF: The result is rounded towards -infinity.
This function can also be called as gmp_div().