Add some generic constants - part 2

This patch fixes a compilation warning on MaxClients not being used, and replace these new constants in various places.
It would make more sense to use charsmax() where it has been replaced now, but this will be done later in another PR for all plugins.
Also, a note about ```get_players```, usually you pass an array of 32, but for the sake to not do ```MAX_PLAYERS - 1```, it should be fine to pass directly ```MAX_PLAYERS``` as last value won't be filled/used. anyway.
Should this be <= MaxClients now?