feat(ui): restore has('gui_running')

Problem:
has('gui_running') is still common in the wild and our answer has
changed over time, causing frustration.
95a6ccbe9f

Solution:
Use stdin_tty/stdout_tty to decide if a UI is (not) a GUI.

src/nvim/api/ui.c

@@ -352,7 +352,6 @@ static void ui_set_option(UI *ui, bool init, String name, Object value, Error *e
     });
 
     stdin_fd = (int)value.data.integer;
-    ui->stdin_fd = (int)value.data.integer;
     return;
   }