SystemServer: Detect spawning user for AcceptSocketConnections services

SystemServer now invokes services with the same uid as the process that
made the request.

This allows the superuser to have a normal GUI workflow. For example,
read and write its own files in TextEditor.

Userland/Services/SystemServer/Service.h

@@ -33,6 +33,8 @@ private:
 
     void spawn(int socket_fd = -1);
 
+    ErrorOr<void> determine_account(int fd);
+
     /// SocketDescriptor describes the details of a single socket that was
     /// requested by a service.
     struct SocketDescriptor {