expand,unexpand: support specifying a trailing tab size

expand,unexpand: support specifying a trailing tab size

* doc/coreutils.texi (expand invocation): Document the feature.
(unexpand invocation): Likewise.
* src/expand-common.c (extend_size): A new global to use
when the last tab stop is prefixed by '/'.
(set_extend_size): A new function to validate and set
the new extend_size global.
(parse_tab_stops): Call set_extend_size() for '/' prefixes.
(finalize_tab_stops): Ensure a single specified '/' is
treated like a standard tabsize, but also ensure that
when '/' is specified with a single other entry that
we process as a list rather than a tab size.
(get_next_tab_stop): Use the tab size if set,
for items after the user specified tab position list.
* tests/misc/expand.pl: Add test cases
* NEWS: Mention the new feature.
Fixes http://bugs.gnu.org/25540