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.