Adapt to new human and xstrtol API.
* src/df.c (long_options): Prepend "--" to long options that
OPT_STR might decode.
* src/du.c (long_options): Likewise.
* src/od.c (long_options): Likewise.
* src/sort.c (long_options): Likewise.
* src/df.c (main): Adjust to new human and xstrtol API.
* src/du.c (main): Likewise.
* src/ls.c (decode_switches): Likewise.
* src/od.c (main): Likewise.
* src/pr.c (first_last_page): Likewise. New argument OPTION.
All callers changed.
* src/sort.c (specify_sort_size): New arg OPTION. All callers
changed. Adjust to new xstrtol API.
* src/system.h (opt_str_storage): New static var.
(OPT_STR, LONG_OPT_STR, OPT_STR_INIT): New macros.