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/isc/netmgr/netmgr.c

@@ -1620,7 +1620,7 @@ isc___nmhandle_get(isc_nmsocket_t *sock, isc_sockaddr_t *peer,
 		if (!atomic_load(&sock->client)) {
 			break;
 		}
-		/* fallthrough */
+		FALLTHROUGH;
 	case isc_nm_tcpsocket:
 	case isc_nm_tlssocket:
 		INSIST(sock->statichandle == NULL);
@@ -2898,7 +2898,7 @@ shutdown_walk_cb(uv_handle_t *handle, void *arg) {
 				isc__nmsocket_reset(sock);
 				return;
 			}
-			/* FALLTHROUGH */
+			FALLTHROUGH;
 		default:
 			isc__nmsocket_shutdown(sock);
 		}