For switch cases where existing comments or code flow logic obviously
intended to fall through, add a gcc-7 /*-fallthrough*/ comment to
silence warnings about this. Some cases which are less obvious are not
converted, so the code does not yet compile clean with
-Wimplicit-fallthrough.
Reported-by: parkerlreed <https://github.com/dxx-rebirth/dxx-rebirth/issues/338>