expand,unexpand: add support for incremental tab stops

expand,unexpand: add support for incremental tab stops

Support --tabs="1,+8" which is equivalent to --tabs="1,9,17,..."
useful for viewing unified diff output with its 1 character
gutter for example.

* doc/coreutils.texi ({expand,unexpand} invocation): Document,
using diff processing as the example.
* src/expand-common.c (set_increment_size): Update the new
increment_size global.
(parse_tab_stops): Handle the new '+' prefix.
(finalize_tab_stops): Verify both '+' and '/' prefixes
are not used together.
* tests/misc/expand.pl: Add test cases.
* NEWS: Mention the new feature.