Accommodate more xstrtol changes.

Accommodate more xstrtol changes.
* src/df.c (long_options): Don't bother prepending "--" to long
options that OPT_STR might decode, as that hack is no longer needed.
(main): Invoke xstrtol_fatal rather than STRTOL_FATAL_ERROR.
* src/du.c (long_options, main): Likewise.
* src/ls.c (decode_switches): Likewise.
* src/od.c (long_options, main): Likewise.
* src/pr.c (first_last_page, main): Likewise.
* src/sort.c (long_options, specify_sort_size): Likewise.
* src/pr.c (first_last_page): Accept option index and option char
instead of an assembled option string.  All callers changed.
* src/sort.c (specify_sort_size): Likewise.
* src/system.h (OPT_STR, LONG_OPT_STR, short_opt_str, OPT_STR_INIT):
Remove.