### Purpose
Fix https://github.com/syncthing/syncthing/issues/9336
The `emitLoginAttempt` function now checks for the presence of an
`X-Forwarded-For` header. The IP from this header is only used if the
connecting host is either on loopback or on the same LAN.
In the case of a host pretending to be a proxy, we'd still have both IPs
in the logs, which should make this much less critical from a security
standpoint.
### Testing
1. directly via localhost
2. via proxy an localhost
#### Logs
```
[3JPXJ] 2025/04/11 15:00:40 INFO: Wrong credentials supplied during API authorization from 127.0.0.1
[3JPXJ] 2025/04/11 15:03:04 INFO: Wrong credentials supplied during API authorization from 192.168.178.5 proxied by 127.0.0.1
```
#### Event API
```
{
"id": 23,
"globalID": 23,
"time": "2025-04-11T15:00:40.578577402+02:00",
"type": "LoginAttempt",
"data": {
"remoteAddress": "127.0.0.1",
"success": false,
"username": "sdfsd"
}
},
{
"id": 24,
"globalID": 24,
"time": "2025-04-11T15:03:04.423403976+02:00",
"type": "LoginAttempt",
"data": {
"proxy": "127.0.0.1",
"remoteAddress": "192.168.178.5",
"success": false,
"username": "sdfsd"
}
}
```
### Documentation
https://github.com/syncthing/docs/pull/907
---------
Co-authored-by: Jakob Borg <jakob@kastelo.net>
c667ada63a