Freedom UDP: Fix some cone uses like STUN,... when address is domain

[patterniha]

although #4816 has no problem and cover most uses,

but it doesn't help for some special cone uses like STUN, when only initial-request-address can be a domain(and using fakedns)

(#4816 didn't break anything, just didn't fix this situation)

///

this PR also fix situations like STUN when only initial-request-address can be a domain.

///

solving other situations when other requests can also be domain requires more effort, needs the implementation of reverse-IP-lookup-map and some changes to "override.go", but it may be useless, because different domains may have the same-IP, so it is impossible to map IP to domain: #2962 (comment)

[RPRX]

Reviewing... 我想了下如果配置中有覆写地址/端口，似乎返回包的来源也没那么重要，可以这么改

[patterniha]

yes, if address is override in config, b.UDP remain nil, so socks-server use (initial)request-address as response-address.

anyway, response-address should be matched with one of request-addresses.

[RPRX]

然后这个 PR 的逻辑是如果返回包的来源和发送包时的去向相同，则修改返回包的来源（可能一样，或改成 domain），LGTM

[patterniha]

if the address before resolving is IP, changedAddr=unchangedAddr
otherwise when resolved-IP is recevied, it convert to domain.

also, for fakedns we have another change(domain <-> fakeIP) and "override.go" handle that, so we should not worry about that.

[RPRX]

@patterniha You've been invited to join Project X organization

[RPRX]

虽然暂时没有仓库的维护权限，但是 @patterniha 展现出来的代码能力是值得认可的，感谢你的每一次贡献

[patterniha]

Thank you very much, it's an honor.

Actually, I saw Xray-core on my people's phones and wanted to help develop it so that I could help my people.

[KobeArthurScofield]

Thank you, the thing works like I've expected before!