Fix backwards compatibility nut_sides arg

* nut_sides must be last argument
* default value = 0. With 120 as default, we cannot set nut_sides to 120
  while simultaneously setting fn to something different from 120.

docs/img_prep/nut_sides.scad

@@ -0,0 +1,15 @@
+/*
+Demo nut_sides argument of nut() module.
+*/
+
+use <threadlib/threadlib.scad>
+include <../../THREAD_TABLE.scad>
+
+type = "M12x0.5";
+turns = 7;
+Douter = 16;
+higbee_arc = 45;
+fn = 16;
+nut_sides = 6;
+
+nut(type, turns, Douter, higbee_arc, fn, THREAD_TABLE, nut_sides);

threadlib.scad

@@ -48,8 +48,8 @@ module bolt(designator, turns, higbee_arc=20, fn=120, table=THREAD_TABLE) {
     };
 };
 
-module nut(designator, turns, Douter, higbee_arc=20, fn=120, nut_sides=120, table=THREAD_TABLE) {
-    nut_sides = nut_sides == 120 ? fn : nut_sides;
+module nut(designator, turns, Douter, higbee_arc=20, fn=120, table=THREAD_TABLE, nut_sides=0) {
+    nut_sides = nut_sides == 0 ? fn : nut_sides;
     union() {
         specs = thread_specs(str(designator, "-int"), table=table);
         P = specs[0]; Dsupport = specs[2];