Safe function to subtract fixed point numbers clamping underflow.


Although fixed point numbers can be subtracted with the normal '-' integer operator, that doesn't provide any protection against overflow. If overflow is a problem, you should use this function instead. It is slower than using integer operators, but if an overflow occurs it will set `errno' and clamp the result, rather than just letting it wrap. Example:
      fixed result;
      /* This will put 4965 into `result'. */
      result = fixsub(itofix(5000), itofix(35));
      /* Sets `errno' and puts -32768 into `result'. */
      result = fixsub(itofix(-31000), itofix(3000));
      ASSERT(!errno); /* This will fail. */
Returns the clamped result of subtracting `y' from `x', setting `errno' to ERANGE if there was an overflow.
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