Use Gnulib’s same-inode module

This is more portable to non-POSIX systems.
However, don’t bother trying to port to systems
where st_ino is not a scalar of type dev_t,
as these systems no longer seem to be active targets
and it’s not worth the maintenance hassle.
* gnulib.modules: Add same-inode, now that we use it
explicitly rather than indirectly.
* src/compare.c (diff_link):
* src/create.c (compare_links, restore_parent_fd):
* src/incremen.c (compare_directory_meta, procdir):
* src/extract.c (dl_compare, repair_delayed_set_stat)
(apply_nonancestor_delayed_set_stat, extract_link)
(apply_delayed_link):
* src/names.c (add_file_id):
* src/system.c (sys_file_is_archive, sys_detect_dev_null_output):
Include same-inode.h, and prefer its macros and functions
to doing things by hand.
* src/create.c (struct link):
* src/extract.c (struct delayed_set_stat, struct delayed_link):
* src/incremen.c (struct directory):
* src/names.c (struct file_id_list):
Rename members to st_dev and st_ino so that SAME_INODE and
PSAME_INODE can be used on the type.  All uses changed.
* src/system.c (sys_compare_links): Remove.
All uses replaced by psame_inode.

src/compare.c

@@ -34,6 +34,7 @@
 #include <alignalloc.h>
 #include <quotearg.h>
 #include <rmt.h>
+#include <same-inode.h>
 #include <stdarg.h>
 
 /* Nonzero if we are verifying at the moment.  */
@@ -251,7 +252,7 @@ diff_link (void)
 
   if (get_stat_data (current_stat_info.file_name, &file_data)
       && get_stat_data (current_stat_info.link_name, &link_data)
-      && !sys_compare_links (&file_data, &link_data))
+      && !psame_inode (&file_data, &link_data))
     report_difference (&current_stat_info,
 		       _("Not linked to %s"),
 		       quote_n_colon (QUOTE_ARG,