DavePack bug fix

"Jano" discovered a leading-slash bug in DavePack, so this has been fixed now. Thanks Jano! The leading-slash was causing the user's afterburner effect to also appear on other multiplayer airplanes. I also had to include a new JSBSim system, called "ab_workaround", that works around a multiplayer limitation, namely that the afterburner state is not sent over the multiplayer protocol. So far only the F-4J and F-4N have been converted, and I could use some testers. Please let me know if the afterburner animations of these two aircraft work properly over multiplayer.