chcon,chgrp,chmod,chown,du,ls: ignore missing files while traversing

chcon,chgrp,chmod,chown,du,ls: ignore missing files while traversing

* src/fts-missing.h: Add helper routines to determine
if an FTS entry is skippable.  I.e., that is traversed
and not specified on the command line.
* src/chcon.c (process_file): Setup to ignore skippable files.
(change_file_context): Ignore missing files if appropriate.
* src/chmod.c (process_file): Setup to ignore skippable files.
Ignore missing files if appropriate.
(mode_changed): Ignore missing files if appropriate.
* src/chown-core.c (change_file_owner): Setup to ignore skippable files.
Ignore missing files if appropriate.
* src/du.c (process_file): Likewise.
* src/ls.c (print_dir): Ignore missing files if appropriate.
(get_link_name): Likewise.
(gobble_file): Likewise.  Also handle the dangling symlink case.
* src/system.h (ignorable_traversal_errno): Match ENOENT for missing
files.
* tests/chcon/traversal-missing.sh: A new test.
* tests/misc/traversal-missing.sh: Likewise.
* tests/local.mk: Reference the new tests.
* NEWS: Mention the bug fix.
Fixes https://bugs.gnu.org/81444