ncurses 6.2 - patch 20210213

ncurses 6.2 - patch 20210213

+ add test/back_ground.c, to exercise the wide-character background
  functions.
+ add a check in _nc_build_wch() in case the background character is a
  wide-character, rather than a new part of a multibyte character.
+ improve tracemunch's coverage of form/menu/panel libraries.
+ improve tracemunch's checking/reporting the type for the first
  parameter, e.g., "WINDOW*" rather than "#1".