cp: when copying to dir use dir-relative names

cp: when copying to dir use dir-relative names

When copying to a directory, use functions like openat to access
the destination files, when such functions are available.  This
should be more efficient and should avoid some race conditions.
* bootstrap.conf (gnulib_modules): Add areadlinkat-with-size,
fchmodat, fchownat, mkdirat, mkfifoat, utimensat.
* src/copy.c (lchown) [!HAVE_LCHOWN]:
* src/copy.c, src/system.h (rpl_mkfifo, mkfifo) [!HAVE_MKFIFO]:
Remove.  All uses removed.
(utimens_symlink): Remove; we shouldn’t have to worry about
those obsolete systems any more.  All uses replaced by utimensat.
* src/copy.c (copy_dir, set_owner, fchmod_or_lchmod, copy_reg)
(same_file_ok, writable_destination, overwrite_ok, abandon_move)
(create_hard_link, src_is_dst_backup, copy_internal, copy):
* src/cp.c (make_dir_parents_private, re_protect):
New args for directory-relative names.  All uses changed.
Continue to pass full names as needed, for diagnostics and for
lower-level functions like qset_acl that do not support
directory-relative names.
* src/copy.c (copy_reg): Prefer readlinkat to lstatat for merely
checking whether a file is a symlink, to avoid EOVERFLOW issues.
(subst_suffix): New function.
(create_hard_link): Accept a null SRC_NAME as meaning that if it
is needed it needs to be constructed from SRC_RELNAME, DST_NAME,
and DST_RELNAME.
(source_is_dst_backup): Use subst_suffix instead of doing it by hand.
(copy_internal): Remember and use directory-relative names instead
of full names.
* src/cp.c (lchown) [!HAVE_LCHOWN]: Remove.  All uses removed.
(must_be_working_directory): New function.
(target_directory_operand): Simply take file name as arg,
and return a file descriptor or negative number on failure;
open with O_DIRECTORY to obtain any file descriptor.
All uses changed.
(target_dirfd_valid): New function.
(do_copy): Use these new functions to obtain a file descriptor
for any target directory, and use directory-relative names
for that directory.
(main): Omit no-longer-needed stat when --target-directory,
as do_copy now does this.
* src/ln.c (O_PATHSEARCH): Move from here ...
* src/system.h: ... to here.
* tests/cp/fail-perm.sh: Adjust to change in diagnostic wording,
and add a test for --no-target-directory.