Add cvars that disable linking of SOLID_NOT entities: [cl|sv]_areagrid_link_SOLID_NOT

This is better than xonotic-data.pk3dir!1122 (closed) and/or trying to be sure we never accidentally do anything that could link a pure entity somewhere, because it works in all cases and is more flexible: for example we could call setorigin on pure entities without causing them to be linked (instead it will unlink them if they're currently linked). Also no macro madness needed.