basic/path-util: allow flags for path_equal_or_files_same

No functional change, just a new parameters and the tests that
AT_SYMLINK_NOFOLLOW works as expected.

src/basic/stat-util.h

@@ -49,7 +49,7 @@ int null_or_empty_fd(int fd);
 int path_is_read_only_fs(const char *path);
 int path_is_os_tree(const char *path);
 
-int files_same(const char *filea, const char *fileb);
+int files_same(const char *filea, const char *fileb, int flags);
 
 /* The .f_type field of struct statfs is really weird defined on
  * different archs. Let's give its type a name. */