tun2socks: fix possible crash when tcp_close() is called from within lwip callbacks. The problem is that tcp_close() may in certain cases immediately free the PCB, and we need to indicate such an event that to  the lwip code calling the callbacks by returning ERR_ABRT, or the code assumes the PCB survived. Fix by always returning  ERR_ABRT from callbacks if tcp_close() gets called. It  appears that there are no ill effects if ERR_ABRT is returned even  though tcp_close() didn't immedietely free the PCB.
tun2socks: fix possible crash when tcp_close() is called from within lwip callbacks.
The problem is that tcp_close() may in certain cases immediately free the PCB, and we need to indicate such an event that to 
the lwip code calling the callbacks by returning ERR_ABRT, or the code assumes the PCB survived. Fix by always returning 
ERR_ABRT from callbacks if tcp_close() gets called. It  appears that there are no ill effects if ERR_ABRT is returned even 
though tcp_close() didn't immedietely free the PCB.

Ernillew/badvpn,Ernillew/badvpn,Ernillew/badvpn,Ernillew/badvpn,Ernillew/badvpn