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.