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/rpz.c

@@ -2715,7 +2715,7 @@ dns_rpz_find_name(dns_rpz_zones_t *rpzs, dns_rpz_type_t rpz_type,
 				found_zbits = nm_data->set.ns;
 			}
 		}
-		/* FALLTHROUGH */
+		FALLTHROUGH;
 
 	case DNS_R_PARTIALMATCH:
 		i = chain.level_matches;