This option makes tar sort the entries of directories that will be
added to an archive according to ORDER (none, name, or order).
Based on proposition by Dick Streefland (https://savannah.gnu.org/patch/?7892).
* src/common.h (savedir_sort_order): New global.
* src/create.c: Pass savedir_sort_order to streamsavedir.
* src/misc.c: Likewise.
* src/tar.c: New option --sort.
* NEWS: Update.
* doc/tar.texi: Document the --sort option.
* doc/tar.1: Likewise.