Add some generic constants

This is similar to what does SM, new constants are added:
* ```MAX_PLAYERS```
* ```MAX_NAME_LENGTH```
* ```MaxClients```

Worth to be noted, redefinition of macros (with different value) is only a warning and overwrites the old value properly.
Also, to avoid ```MaxClients``` to be modifed by xvar natives; a simple check is probably enough as I don't think more such public constants to be added again.

This patch is most likely harmful.