ncurses 6.1 - patch 20190623

+ improve the tabs.1 manual page to distinguish the PWB/Unix and 7th
  Edition versions of the tabs utility.
+ add configure check for getenv() to work around implementation shown
  in Emscripten #6766, use that to optionally suppress START_TRACE
  macro, whose call to getenv() may not work properly (report by Ilya
  Ig Petrov).
+ modify initialization functions to avoid relying upon persistent
  data for the result from getenv().
+ update config.guess, config.sub