Add natives from YSF

[Jedrzej94]

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native LoadFilterScript(const scriptname[]); // difference between "rcon loadfs": Return -> True if success, false if not
native UnLoadFilterScript(const scriptname[]); // ^
native GetFilterScriptCount();
native GetFilterScriptName(id, name[], len = sizeof(name));

the return value of LoadFilterScript is the most important part. SendRconCommand always returns 1.

[Jedrzej94]

https://github.com/IS4Code/YSF/blob/50a861667370a07aa2c52b3a2fd03c0db566e1c1/src/CFunctions.cpp#L153

[Jedrzej94]

Please make sure that the message: Filterscript 'xxx' loaded. is not generated when using these functions.