cat: fallback on EISDIR with copy_file_range(2)

The filesystem may support reading directories directly
when security.bsd.allow_read_dir is set.

MFC after:	1 week
PR:		275099

bin/cat/cat.c

@@ -281,7 +281,8 @@ scanfiles(char *argv[], int cooked __unused)
 		} else {
 #ifndef BOOTSTRAP_CAT
 			if (in_kernel_copy(fd) == -1) {
-				if (errno == EINVAL || errno == EBADF)
+				if (errno == EINVAL || errno == EBADF ||
+				    errno == EISDIR)
 					raw_cat(fd);
 				else
 					err(1, "stdout");