fts: fix cloexec races

fts: fix cloexec races

* lib/fts.c [!_LIBC]: Do not include dirent--.h, unistd--.h, cloexec.h.
(opendirat, diropen): Use O_CLOEXEC instead of set_cloexec_flag.
(fts_build): Use F_DUPD_CLOEXEC rinstad of set_cloexec_flag.
(fd_ring_check): Set cloexec flag on new file descriptors.
(fts_build, fd_ring_check): While we’re at it, make sure the
resulting file descriptor is not 0, 1, or 2, since that is easy.
* modules/fts (Depends-on): Remove cloexec, dirent-safer, dup,
fcntl-safer, unistd-safer.  Add fcntl.