Fixes #18 - vpnc-script-win.js not work in Windows XP.

The "route add" command produces an empty output, the file "vpnc.out" is empty (0 bytes) and the script crash (see [here](http://stackoverflow.com/questions/1587591/getting-error-on-while-reading-a-blank-text-file)). With this patch, the script run fine in Windows XP SP2.

Note: In Windows XP the command "netsh interface ipv4 set subinterface ... mtu" does not exist (see [here](http://superuser.com/questions/37686/how-to-tell-what-mtu-is-being-used-in-windows-xp)). For me, works this: only once, I setup the MTU in Control Panel --> Network --> OpenConnect Adapter --> Advanced tab --> MaxFrameSize --> 1406.