Replace instances of 'subsystem' with 'client'.

'client' is a more general term, and is self explanatory. 'subsystem' somewhat conveys it's meaning, but could be misunderstood as an internal component/library, and needlessly creates a new word. THAT BEING SAID; subsystem should be kept as a general descriptor of the game mods, "The FMCB Minecraft subsystem" makes a lot more sense than "The FMCB Minecraft Client" (at least from the view of an end-user), as client would infer a launcher/coremod. Any instances in code documentation, some instances in the wiki, etc, should be replaced. If needed, sentence structure/grammar should of course be modified to insure the new term makes sense in context.