Disable performing Xauth authentication on rekey of session.

For your consideration,

Some clients don't handle Xauth correctly and prompt for authentication causing the session to be dropped (MacOS Mountain Lion builtin VPN client is one such client).  Disabling Xauth on during rekey solves this problem.  I have tested this patch with IOS 5.x and 6.x,  Cisco VPN 3000 client, and  Shrew Soft VPN client.  

Mac OS Mountain Lion now consistently holds a session for over 8 hours (the limit of my testing) and has not shown any negative effects to the other clients.