Treat fishy-looking hard links like fishy-looking symlinks.

(struct delayed_set_stat): Rename after_symlinks
member to after_links.  All uses changed.
(struct delayed_link): Renamed from struct delayed_symlink.
All uses changed.  New member is_symlink.
(delayed_link_head): Renamed from delayed_symlink_head.  All uses
changed.
(create_placeholder_file): New function, taken from extract_symlink.
(extract_link): Create placeholders for fishy-looking hard links.
(extract_symlink): Move code into create_placeholder_file.
(apply_delayed_links): Renamed from apply_delayed_symlinks.
All uses changed.  Create both hard links and symlinks.