Prepare for Bullet 2.86 release, update version to 2.86 (release will be tagged soon)

protect some C-API methods against negative dof indices
add triangle mesh import from MJCF files
update to recent pybullet quickstart guide pdf

src/LinearMath/btSerializer.h

@@ -505,7 +505,7 @@ public:
 
 			buffer[9] = '2';
 			buffer[10] = '8';
-			buffer[11] = '5';
+			buffer[11] = '6';
 
 		}