Make fts (in FTS_CWDFD mode) more efficient by caching a few open

Make fts (in FTS_CWDFD mode) more efficient by caching a few open
file descriptors.  This also averts a failure on systems with
native openat support when a traversed directory lacks "x" access.
* lib/fts_.h: Include "i-ring.h"
(struct FTS) [fts_fd_ring]: New member.
* lib/fts.c (RESTORE_INITIAL_CWD): Also call fd_ring_clear.
(FCHDIR): Add parentheses.
(fd_ring_check, fd_ring_print) [!FTS_DEBUG]: Define away.
(cwd_advance_fd): Add a 3rd parameter.  Adjust all callers.
When descending, rather than simply closing the previous
fts_cwd_fd value, push that file descriptor onto the ring.
(same_fd, fd_ring_print, fd_ring_check) [FTS_DEBUG]: New functions.
(fts_open): Initialize the new fd_ring member.
(fts_close): Clear the ring.
(fts_safe_changedir): When possible, use our new fd_ring to skip
the diropen and fstat and dev/ino comparison that would normally
accompany a virtual `chdir ("..")'.

* modules/fts (Depends-on): Add i-ring.
* modules/i-ring: New module.
* lib/i-ring.c, lib/i-ring.h, lib/i-ring-test.c: New files.
* m4/i-ring.m4: New file.