Now that fts no longer changes the current working directory, adjust

Now that fts no longer changes the current working directory, adjust
its clients accordingly -- note that du.c uses fts but doesn't need
any adjustment, since it doesn't operate on the actual files,
but rather just uses the stat buffers provided by fts.

Include "openat.h".
(process_file): Use chmodat (fts->fts_cwd_fd,... in place of chmod (...