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.

lib/dns/validator.c

@@ -529,7 +529,7 @@ fetch_callback_ds(isc_task_t *task, isc_event_t *event) {
 			goto unexpected;
 		}
 
-	/* FALLTHROUGH */
+		FALLTHROUGH;
 	case ISC_R_SUCCESS:
 		if (trustchain) {
 			/*