UDP fullcone NAT does not work on v2rayNG

[mmmray]

Using freedom outbound on v2rayNG does not support UDP fullcone (one source port, many dest IP) Affects all recent versions.

Reference: #3734

[mmmray]

In the past there were multiple related "type safety" issues with custom system dialers, for example #3559 -- it feels like a larger refactor might be appropriate.

[mmmray]

I guess creating this issue was not really necessary. 2dust/AndroidLibXrayLite#57 got merged now.