maint: avoid warnings from upcoming GCC8

* src/transform.c (_single_transform_name_to_obstack): Mark with
FALLTHROUGH statement rather than /* FALL THROUGH */ comment.
Only the former works with gcc-8.
* src/extract.c (maybe_recoverable): Call abort to tell gcc-8 that
this code is unreachable.

src/extract.c

@@ -788,6 +788,7 @@ maybe_recoverable (char *file_name, bool regular, bool *interdir_made)
 	case UNLINK_FIRST_OLD_FILES:
 	  break;
 	}
+      abort (); /* notreached */
 
     case ENOENT:
       /* Attempt creating missing intermediate directories.  */

src/transform.c

@@ -550,7 +550,7 @@ _single_transform_name_to_obstack (struct transform *tf, char *input)
 			default:
 			  break;
 			}
-		      /*FALL THROUGH*/
+		      FALLTHROUGH;
 
 		    case ctl_upcase:
 		    case ctl_locase: