normalize use of stdio functions taking a stream

We should never use stdio functions that use stdout implicitly. Saving a
few characters isn't worth the inconsistency. Too, using the forms such
as `fwprintf()` which take an explicit stream makes it easier to find
the places we write to stdout versus stderr.

Fixes #3728

src/screen.cpp

@@ -661,7 +661,7 @@ static bool test_stuff(screen_t *scr)
         int c = getchar();
         if (c != EOF) break;
     }
-    wprintf(L"Bye\n");
+    fwprintf(stdout, L"Bye\n");
     exit(0);
     while (1) sleep(10000);
     return true;