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

examples/SharedMemory/PhysicsServerExample.cpp

@@ -1119,7 +1119,7 @@ public:
 
 		if (args.CheckCmdLineFlag("norobotassets"))
 		{
-//			gCreateDefaultRobotAssets = false;
+			gCreateDefaultRobotAssets = false;
 		}