mv: when installing to dir use dir-relative names

mv: when installing to dir use dir-relative names

When the destination for mv is 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.
Likewise for 'install'.
* src/cp.c (must_be_working_directory, target_directory_operand)
(target_dirfd_valid): Move from here ...
* src/system.h: ... to here, so that install and mv can use them.
Make them inline so GCC doesn’t complain.
* src/install.c (lchown) [HAVE_LCHOWN]: Remove; no longer needed.
(need_copy, copy_file, change_attributes, change_timestamps)
(install_file_in_file, install_file_in_dir):
New args for directory-relative names.  All uses changed.
Continue to pass full names as needed, for diagnostics and for
lower-level functions that do not support directory-relative names.
(install_file_in_dir): Update *TARGET_DIRFD as needed.
(main): Handle target-directory in the new, cp-like way.
* src/mv.c (remove_trailing_slashes): Remove static var; now local.
(do_move): New args for directory-relative names.  All uses changed.
Continue to pass full names as needed, for diagnostics and for
lower-level functions that do not support directory-relative names.
(movefile): Remove; no longer needed.
(main): Handle target-directory in the new, cp-like way.
* tests/install/basic-1.sh:
* tests/mv/diag.sh: Adjust to match new diagnostic wording.