Fix #110253: Regression: Impossible to extrude a main bone in rigging mode

Caused by 0b7d8a20bf
Above commit broke the operation because of this change: `if (do_extrude != 0)`
Earlier True/False/2 were used to determine the point of extrude.
Use enum to fix this and to improve the code readability.

@@ -1413,2 +1412,4 @@
   Object **objects = BKE_view_layer_array_from_objects_in_edit_mode_unique_data(
       scene, view_layer, CTX_wm_view3d(C), &objects_len);
+
+    enum ExtrudePoint {

@@ -1420,2 +1426,3 @@
     EditBone *newbone = nullptr, *ebone, *flipbone, *first = nullptr;
-    int a, totbone = 0, do_extrude;
+    int a, totbone = 0;
+    ExtrudePoint do_extrude;