dirent-util: add FOREACH macro for iterating through getdents64() buffers

We already have a similar loop twice, let's make it easier to read via
an iteration macro.

(The new macro is a bit more careful even, as it verifies the full
dirent fits into the remaining buffer when returning it)

src/basic/recurse-dir.h

@@ -71,7 +71,7 @@ typedef struct DirectoryEntries {
         size_t n_entries;
         struct dirent** entries;
         size_t buffer_size;
-        uint8_t buffer[] _alignas_(struct dirent);
+        struct dirent buffer[];
 } DirectoryEntries;
 
 int readdir_all(int dir_fd, RecurseDirFlags flags, DirectoryEntries **ret);