+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
+