Mod-created CVars are now destroyed when using the "restart" CCMD

Any CVars created by mods weren't destroyed when the restart CCMD was executed, so if file(s) with custom CVars were reloaded, Zandronum would throw a fatal error telling the user that the CVars already exist. This especially makes the internal server browser unusable since it uses this CCMD. Transplanting the changeset from GZDoom was very easy.