Default to use "127.0.0.1" for "listen" field

[YanB25]

Currently, v2ray uses "0.0.0.0" as the default value for "listen" field. I.e.

"inbounds" : [
    {
     // if "listen" field is not provided
     // default to this value
     // "listen" : "0.0.0.0",
    },

It's potentially insecure. In fact, my server is hacked somehow by allowing "0.0.0.0" as inbounds.

Better to use "127.0.0.1" as the default value.