(LS_MINOR_PROBLEM, LS_FAILURE): New constants.

(LS_MINOR_PROBLEM, LS_FAILURE): New constants.
All uses of EXIT_FAILURE replaced with LS_FAILURE, unless
specified below.
(main): Initialize exit failure to LS_FAILURE.
(print_dir, gobble_file, get_link_name, xstrcoll):
Set exit status to LS_MINOR_PROBLEM if the failure is minor.
(print_dir): Do not give up on entire directory merely because readdir
returns EOVERFLOW.
(usage): Explain exit status.