Bring back placeholders

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.