(struct delayed_symlinks, extract_archive, apply_delayed_symlinks):

(struct delayed_symlinks, extract_archive, apply_delayed_symlinks):
Support hard links to symbolic links.

(struct delayed_symlink): Remove 'names' member, replacing it with
'sources' and 'target' member.  All uses changed.

(struct string_list): New type.

(delayed_set_stat, extract_archive): Use offsetof when computing sizes
for struct hack; this avoids wasted space in some cases.

(extract_archive): Fix test for absolute pathnames and/or "..".
Use link_error to report errors for links.
Remove redundant trailing '/' at "really_dir", for all uses, not
just before invoking mkdir.
If overwriting old files, do not worry so much about existing directories.
Fix mode computation in the case where the directory exists.

(apply_delayed_symlinks): If we can't make a hard link to a symbolic
link, make a copy of the symbolic link.