[Ncurses] Bug: tabs should support stop specification like "+n"

[Philip Ganchev]

The manual for the "tabs" program says:
   ...
       Use a '+' to treat a number as an increment relative to the
previous value, e.g.,

              tabs 1,+5,+5,+5,+5
       which is equivalent to the 1,6,11,16,21 example.
   ...


But when I try that, I get

   tabs: unexpected character found '+

I checked the source code for ncurses-5.7-20101009. Function
legal_tab_list() accepts only digits and comma.