xrt,comp,ipc...: Name changes related to "system compositor".

Now called a "system composition manager" (syscompmgr) because it manages composition, but is not itself a compositor (xrt_compositor). This is a naming quirk that had really thrown me for a long time and kept me from understanding how things worked. Along with this name change, I've also updated the docs as required, clarifying the roles of system composition things where it made sense.

• Undecided: is xrt_system_composition_manager xscm or xsyscm? xrt_system_compositor was inconsistently xsysc and xsc in various places.