copy: check for vulnerable target dirs

copy: check for vulnerable target dirs

* NEWS, doc/coreutils.texi (Target directory): Document this.
* src/cp.c, src/install.c, src/ln.c, src/mv.c: Include targetdir.h.
(target_directory_operand): Use the new targetdir_operand_type
function to check for vulnerable target directories.
* src/cp.c (stat_target_operand): New function.
(target_directory_operand, do_copy): Use it.
* src/local.mk (noinst_HEADERS): Add src/targetdir.h.
(src_ginstall_SOURCES, src_cp_SOURCES, src_ln_SOURCES)
(src_mv_SOURCES): Add src/targetdir.c.
* src/targetdir.c, src/targetdir.h: New files.
* tests/mv/vulnerable-target.sh: New test.
* tests/local.mk (all_root_tests): Add it.