`fopen`/`ti_Open` does not support `rb+` and etc

[ZERICO2005]

fopen currently wraps ti_Open. Currently, ti_Open supports r, a, w, r+, a+, and w+.

I am not exactly sure, but I think ti_Open correctly implements rb, ab, and wb since I believe they would behave no different compared to r, a, and w on this platform. However, it only checks if (str[1] == '+'), so rb+ would be treated as r instead of r+

One fix for this is to modify the fileioc logic to check if (str[2] == '+') so rb+ would be interpreted as r+

https://en.cppreference.com/w/c/io/fopen

[ZERICO2005]

Actually, this would only be an issue with fopen, and not ti_Open unless we decide to add support for the b flag in ti_Open (which we currently do not). So the fix would just be in the implementation of fopen.