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/log.c

@@ -1798,7 +1798,7 @@ isc_log_doit(isc_log_t *lctx, isc_logcategory_t *category,
 				}
 				channel->flags &= ~ISC_LOG_OPENERR;
 			}
-			/* FALLTHROUGH */
+			FALLTHROUGH;
 
 		case ISC_LOG_TOFILEDESC:
 			fprintf(FILE_STREAM(channel), "%s%s%s%s%s%s%s%s%s%s\n",