Introduce int_rounding function

[S-H-GAMELINKS]

int_floor, int_ceil and int_truncate has similar code.

    int ndigits;

    if (!rb_check_arity(argc, 0, 1)) return num;
    ndigits = NUM2INT(argv[0]);
    if (ndigits >= 0) {
        return num;
    }

I thought better to merge these code to function like a int_rounding.

static VALUE
int_rounding(int argc, VALUE *argv, VALUE num, VALUE func(VALUE, int))
{
    int ndigits;

    if (!rb_check_arity(argc, 0, 1)) return num;
    ndigits = NUM2INT(argv[0]);
    if (ndigits >= 0) {
        return num;
    }
    return func(num, ndigits);   
}