ncurses 6.3 - patch 20220813

ncurses 6.3 - patch 20220813

+ modify delscreen to more effectively delete all windows on the given
  screen.
+ amend portability note for delwin in manual page.
+ adapt test/test_delwin.c from example by Bill Gray.
+ account for prescreen data if freeing leaks in pthread-configuration
+ split-out _nc_set_read_thread(), to reduce compiler warnings about
  pthread_self(), which may/may not be a weak symbol.
+ improve pthread-configuration for test/worm.c