(gobble_file): Change arg name to be command_line_arg

(gobble_file): Change arg name to be command_line_arg
rather than explicit_arg, for consistency with copy.c.
(extract_dirs_from_files): Remove ignore_dot_and_dot_dot arg, since
it is deducible from dirname arg.  All callers changed.
(extract_dirs_from_files, print_dir, queue_directory):
Add command_line_arg arg.  All callers changed.
(struct pending): Add command_line_arg member.
(main): Use NULL rather than 0 when appropriate.
(set_exit_status, file_failure): New functions.
(queue_directory): Store command_line_arg into new structure.
(print_dir, gobble_file, get_link_name):
Use file_failure to report problems in accessing files,
so that the exit status is set consistently.
(print_dir): Simplify readdir failure code yet again.
If closedir fails, report "closing directory" rather than "reading
directory" failure.
(xstrcoll): Use set_exit_status to set status on failure.