tar: use birthtime rather than ctime when checking identity

Without this fix, tar would sometimes mishandle the extraction of
hard links to symbolic links.  Problem reported in
<http://lists.gnu.org/archive/html/bug-tar/2011-06/msg00000.html>
by Andreas Stolcke.
* src/extract.c (struct delayed_link): Change ctime member to birthtime.
(same_birthtime): New function.
(create_placeholder_file, extract_link, apply_delayed_links):
Use it to compare birthtimes (when available) instead of ctime.