stat-util: rename files_same() → inode_same()

Let's be more accurate about what this function does: it checks whether
the underlying reported inode is the same. Internally, this already uses
a better named stat_inode_same() call, hence let's similarly name the
wrapping function following the same logic.

Similar for files_same_at() and path_equal_or_same_files().

No code changes, just some renaming.

src/basic/process-util.c

@@ -946,7 +946,7 @@ int pid_from_same_root_fs(pid_t pid) {
 
         root = procfs_file_alloca(pid, "root");
 
-        return files_same(root, "/proc/1/root", 0);
+        return inode_same(root, "/proc/1/root", 0);
 }
 
 bool is_main_thread(void) {