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/masterdump.c

@@ -703,7 +703,7 @@ rdataset_totext(dns_rdataset_t *rdataset, const dns_name_t *owner_name,
 				isc_buffer_putstr(target, KEYDATA);
 				break;
 			}
-		/* FALLTHROUGH */
+			FALLTHROUGH;
 		default:
 			if ((ctx->style.flags & DNS_STYLEFLAG_UNKNOWNFORMAT) !=
 			    0) {