Use __attribute__((fallthrough)) rather than magic GCC comments.

GCC added an implicit-fallthrough warning a while back, where it
would complain if you had a nontrivial "case:" block that didn't end
with break, return, or something like that.  Clang recently added
the same thing.

GCC, however, would let you annotate a fall-through as intended by
any of various magic "/* fall through */" comments.  Clang, however,
only seems to like "__attribute__((fallthrough))".  Fortunately, GCC
accepts that too.

A previous commit in this branch defined a FALLTHROUGH macro to do
the right thing if GNUC is defined; here we replace all of our "fall
through" comments with uses of that macro.

This is an automated commit, made with the following perl one-liner:

  #!/usr/bin/perl -i -p
  s#/\* *falls? ?thr.*?\*/#FALLTHROUGH;#i;

src/feature/client/entrynodes.c

@@ -2263,7 +2263,7 @@ entry_guards_note_guard_success(guard_selection_t *gs,
       break;
     default:
       tor_assert_nonfatal_unreached();
-      /* Fall through. */
+      FALLTHROUGH;
     case GUARD_CIRC_STATE_USABLE_IF_NO_BETTER_GUARD:
       if (guard->is_primary) {
         /* XXXX #20832 -- I don't actually like this logic. It seems to make

src/feature/control/control.c

@@ -4934,7 +4934,7 @@ handle_control_add_onion(control_connection_t *conn,
   case RSAE_BADAUTH:
     connection_printf_to_buf(conn, "512 Invalid client authorization\r\n");
     break;
-  case RSAE_INTERNAL: /* FALLSTHROUGH */
+  case RSAE_INTERNAL: FALLTHROUGH;
   default:
     connection_printf_to_buf(conn, "551 Failed to add Onion Service\r\n");
   }

src/feature/dirclient/dirclient.c

@@ -1369,7 +1369,7 @@ directory_initiate_request,(directory_request_t *request))
       case 1:
         /* start flushing conn */
         conn->base_.state = DIR_CONN_STATE_CLIENT_SENDING;
-        /* fall through */
+        FALLTHROUGH;
       case 0:
         /* queue the command on the outbuf */
         directory_send_command(conn, 1, request);

src/feature/dirparse/parsecommon.c

@@ -222,7 +222,7 @@ token_check_object(memarea_t *area, const char *kwd,
                      kwd, crypto_pk_num_bits(tok->key));
         RET_ERR(ebuf);
       }
-      /* fall through */
+      FALLTHROUGH;
     case NEED_KEY: /* There must be some kind of key. */
       if (!tok->key) {
         tor_snprintf(ebuf, sizeof(ebuf), "Missing public key for %s", kwd);

src/feature/relay/dns.c

@@ -548,7 +548,7 @@ send_resolved_cell,(edge_connection_t *conn, uint8_t answer_type,
         answer_type = RESOLVED_TYPE_ERROR;
         /* We let this fall through and treat it as an error. */
       }
-      /* Falls through. */
+      FALLTHROUGH;
     case RESOLVED_TYPE_ERROR_TRANSIENT:
     case RESOLVED_TYPE_ERROR:
       {

src/feature/rend/rendclient.c

@@ -819,7 +819,7 @@ rend_client_report_intro_point_failure(extend_info_t *failed_intro,
         log_warn(LD_BUG, "Unknown failure type %u. Removing intro point.",
                  failure_type);
         tor_fragile_assert();
-        /* fall through */
+        FALLTHROUGH;
       case INTRO_POINT_FAILURE_GENERIC:
         rend_cache_intro_failure_note(failure_type,
                                       (uint8_t *)failed_intro->identity_digest,