Add FALLTHROUGH macro for __attribute__((fallthrough))

Gcc 7+ and Clang 10+ have implemented __attribute__((fallthrough)) which
is explicit version of the /* FALLTHROUGH */ comment we are currently
using.

Add and apply FALLTHROUGH macro that uses the attribute if available,
but does nothing on older compilers.

In one case (lib/dns/zone.c), using the macro revealed that we were
using the /* FALLTHROUGH */ comment in wrong place, remove that comment.

doc/dev/style.md

@@ -145,7 +145,7 @@ Good:
 The following lint and lint-like comments should be used where appropriate:
 
     	/* ARGSUSED */
-    	/* FALLTHROUGH */
+    	FALLTHROUGH;
     	/* NOTREACHED */
     	/* VARARGS */