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.

bin/dnssec/dnssec-dsfromkey.c

@@ -434,14 +434,14 @@ main(int argc, char **argv) {
 			}
 			break;
 		case 'F':
-		/* Reserved for FIPS mode */
-		/* FALLTHROUGH */
+			/* Reserved for FIPS mode */
+			FALLTHROUGH;
 		case '?':
 			if (isc_commandline_option != '?') {
 				fprintf(stderr, "%s: invalid argument -%c\n",
 					program, isc_commandline_option);
 			}
-		/* FALLTHROUGH */
+			FALLTHROUGH;
 		case 'h':
 			/* Does not return. */
 			usage();