ln: use linkat and symlinkat

ln: use linkat and symlinkat

Open a target directory and use its file descriptor in linkat,
symlinkat, etc. syscalls, instead of constructing long file names
by concatenating the target directory name to a basename.
This avoids O(N²) behavior with ‘ln F1 F2 ... Fn DIR’ when DIR is
a long file name with many slashes.  It also avoids some races if
DIR is renamed while ln is running.
* bootstrap.conf (gnulib_modules): Add openat-safer.
* src/ln.c: Include fcntl-safer.h.
(O_PATHSEARCH): New constant.
(errno_nonexisting, target_directory_operand): Remove; no longer used.
(atomic_link, do_link): New arg DESTDIR_FD.  All uses changed.
(do_link): New arg DEST_BASE.  All uses changed.
(main): Open target directory and use its file descriptor
as DESTDIR_FD.