Bring back placeholders

They can still be useful if -h is used.  See Pavel Cahyna in:
https://lists.gnu.org/r/bug-tar/2025-11/msg00026.html
while we’re at it bring them back if -P is used,
as they can still be useful there too.
* src/extract.c (HAVE_BIRTHTIME, BIRTHTIME_EQ):
Bring back these macros.
(struct delayed_link, struct string_list):
Bring back these structs.
(delayed_link_table, delayed_link_head, delayed_link_tail):
Bring back these static vars.
(dl_hash, dl_compare, find_direct_ancestor)
(find_delayed_link_source, create_placeholder_file)
(apply_delayed_link, apply_delayed_links):
Bring back these static functions.
(mark_metadata_set): Rename from mark_after_links.  All uses changed.
(extract_link, extract_symlink):
Create placeholders as before, except only if -P or -h are used.
(extract_finish): Deal with delayed links, as before.

NEWS

@@ -1,4 +1,4 @@
-GNU tar NEWS - User visible changes. 2025-11-13
+GNU tar NEWS - User visible changes. 2025-11-26
 Please send GNU tar bug reports to <bug-tar@gnu.org>
 
 version 1.35.90 (git)
@@ -62,10 +62,11 @@ option.
 
 ** Sparse files are now read and written with larger blocksizes.
 
-** When extracting, tar no longer creates empty placeholder files
+** When extracting and neither --absolute-names (-P) nor --dereference
+   (-h) is used, tar no longer creates empty placeholder files
    that are later replaced by symbolic links.  The placeholders are no
-   longer needed now that tar no longer follows symbolic links to
-   targets outside the working directory.
+   longer needed now that tar by default no longer follows symbolic
+   links to targets outside the working directory.
 
 
 version 1.35 - Sergey Poznyakoff, 2023-07-18