Handle d_type == DT_UNKNOWN correctly

As documented in the man-page, readdir() may return a directory entry with
d_type == DT_UNKNOWN. This must be handled for regular filesystems.

dirent_ensure_type() is available to set d_type if necessary. Use it in
some more places.

Without this systemd will fail to boot correctly with nfsroot and some
other filesystems.

Closes #13609

src/shared/bootspec.c

@@ -475,6 +475,7 @@ static int boot_entries_find_unified(
                 _cleanup_free_ char *j = NULL, *osrelease = NULL, *cmdline = NULL;
                 _cleanup_close_ int fd = -1;
 
+                dirent_ensure_type(d, de);
                 if (!dirent_is_file(de))
                         continue;