cvar overhaul

[Razish]

• move all cvar declarations to *_xcvar.h (e.g. bot_minPlayers)
• expose cvar_t directly in modcode
• push updates from engine -> modcode instead of modcode polling
  • Register() should have optional Update() func
  • store list of update funcs in cvar_t? think about cgame registering then unloading
• separate "owning" vs "view" ptrs to cvars, owning registration should overwrite default value, description, flags etc)
• no runtime Cvar_Get in engine (e.g. renderer, server/bot_debug)
  • refcount cvar_t registration, to track non-owning registrations
    e.g. cl_running is registered in renderer (rd-vanilla/) but owned by engine (qcommon/)
• only save modified or user-created cvars
  • can later add "profiles" which represents state of all cvars
    "configs" are an additive series of commands, not state persistence
    ...which basically means a "profile" is cvar_restart; exec myChangedVars.cfg anyway