Add a safe wrapper for write and fix some warnings

1. Add a function writeall and make swrite wrap that function. Use either writeall or swrite, depending on whether we want to exit on errors or not.
2. Fix warnings when compiling with a higher optimisation level.
(CFLAGS ?= -pipe -O3 -march=native -mtune=native -freorder-blocks-and-partition)

Signed-off-by: hwangcc <hwangcc@csie.nctu.edu.tw>

src/click.c

@@ -46,6 +46,9 @@ static bool tiling_resize_for_border(Con *con, border_t border, xcb_button_press
         case BORDER_BOTTOM:
             search_direction = D_DOWN;
             break;
+        default:
+            assert(false);
+            break;
     }
 
     bool res = resize_find_tiling_participants(&first, &second, search_direction);